Plasma panel based ionizing radiation detector

ABSTRACT

A radiation detector is formed from a plasma panel that includes a front substrate, and a back substrate that forms a generally parallel gap with the front substrate. X (column) and Y (row) electrodes are coupled by gas discharge events to define one or more pixels. Impedances are coupled to the X and Y electrodes, and a power supply is coupled to one or both types of electrodes. Discharge event detectors are coupled to the impedances.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/928,331, filed Oct. 30, 2007, which is a continuation of U.S. patentapplication Ser. No. 11/155,660 filed Jun. 20, 2005, which claimspriority to U.S. Provisional Patent Application No. 60/580,931 filedJun. 19, 2004, the specification of which is herein incorporated byreference.

FIELD OF THE INVENTION

One embodiment of the present invention is directed to the detection andimaging of ionizing radiation. More particularly, one embodiment of thepresent invention is directed to a plasma panel based apparatus for thedetection and imaging of ionizing radiation.

BACKGROUND INFORMATION

Many useful applications, such as the detection of radioactive materialand computer-assisted tomography (“CAT”), rely on the detection ofphoton radiation, known as X-ray and/or gamma-ray radiation. Both ofthese types of high-energy photon radiation cause ionization and for thepurposes of this disclosure the two terms, X-ray and gamma-ray, are usedinterchangeably. In terms of the detection of such ionizing radiation,the spectral region of greatest interest for most applications generallyfalls between the energies of about 20 to 2,000 keV (i.e., 0.02 to 2MeV).

In the above spectral range of interest, the primary types ofinteraction are the photoelectric and Compton effects. The relativecontribution from each can be determined in quantitative fashion apriori via the combination of the incident photon energy and the atomicnumber (i.e., Z-number) of the interacting atom. The photoelectriceffect describes a single atomic absorption, whereas the Compton effectdescribes an inelastic scattering collision that simultaneously resultsin a Compton recoil electron and a Compton scattered photon. The lattercan be inelastically scattered again and again, until the photon eitherexits or is “absorbed” by the interacting media. Of the two processes,the primary basis for the majority of known ionizing radiation detectorsused in imaging applications at photon energies up to at least 200 keVis the photoelectric effect, which causes the initial production of asingle “free-electron” and a corresponding positive atomic ion.

In order to detect ionizing electromagnetic radiation, several knownsensing devices are commonly used. One of the earliest known electronicdevices is the ionization chamber. Detection of radiation in anionization chamber, such as a Geiger-Mueller (“GM”) tube, is based uponelectrical conductivity induced in an inert gas (usually containingargon and neon) as a consequence of ion-pair formation.

More recent and sensitive variations of sensing devices includehigh-pressure xenon ionization chambers, such as the tube disclosed inG. P. Lasche' et al., “Detection Sensitivity for Special NuclearMaterials with an Advanced High-Pressure Xenon Detector and RobustFitting Analysis”, IEEE Trans. Nucl. Sci., 48 (2001), pp. 325-32, whichis designed for portal inspection to detect the presence of ²³⁵U and²³⁹Pu radionuclides. However, these devices are very expensive and likea GM-tube have no internal pixel structure, so cannot count“simultaneous” electrons created in different spatial regions of thetube, nor be internally configured to function as an imaging detector.

In contrast, a gas proportional scintillation counter (“GSPC”) is animaging device in which gaseous interaction primarily with low energyradiation in a high-voltage electric field causes secondary VUV photonsthat are detected by VUV-sensitive, photodiodes or photomultiplier(“PM”) tubes. Some disadvantages of GSPC's are their limited energydetection range, required use of ultrahigh-vacuum technology andultrahigh-purity gases, and very short device lifetimes as measured inmonths.

For thermal (i.e., slow, 2200-m/s) neutron detection, thetwo-dimensional microstrip gas chamber (“MSGC”) is probably the mostcommon known detector, although other related gas-based detectors suchas multiwire proportional counters (“MWPC”), multitube positionsensitive detectors (“PSD”), and GSPC's are also commonly used. However,these two-dimensional detectors are generally suitable only for largeresearch laboratories that can support highly specialized detectorgroups, as they are often custom-built and can be difficult to maintain.In addition, they all require ultrahigh purity gas mixtures composedtypically of ³He or ¹⁰BF₃ as the neutron converter and CF₄ as thequencher, and operate at positive pressures of about 3 to 20atmospheres, presenting a potentially explosive hazard.

Several other known gas detector configurations have receivedconsiderable attention over the past few years including: gas electronmultipliers (“GEM”), microgap chambers (“MGC”), and various combinationsof MSGC and GEM detectors such as multiple-GEM, Micromegas andMICROMEGEM. The GEM structures operate in tandem with MSGC's to improveelectron gain by charge pre-amplification.

The above-described gas detector configurations were developed primarilyfor use in detecting either low energy radiation (i.e. less than 10keV), or very high energy particles (e.g., 300 MeV to 10 GeV pions) inparticle accelerators. With regard to the various MSGC configurations(including the MSGC-GEM), they are designed to operate in the“Proportional Region” of the gas ionization curve, having a typical gasavalanche multiplication gain of ˜10⁴.

Currently, the most effective radiation detector is generally consideredto be a scintillation counter. The basic scintillation counter consistsprimarily of two components—a scintillation plate or crystal, opticallycoupled to a photomultiplier tube or a silicon photodiode. Thescintillation plate or crystal contains phosphor type material thatproduces visible (or ultraviolet) photons upon the occurrence of anabsorption/scattering event caused by incident ionizing radiation. Lightfrom the scintillation material, which commonly is NaI(TI), istransmitted to the photocathode of the photomultiplier, which, through aseries of dynodes, amplifies the electrical signal.

Compared to a GM-tube that can have a “dead-time” on the order of 100 μs(microseconds) between counting events, during which time any responseto radiation is impossible, a scintillation detector generally has adead-time of about 1 μs or less. Another advantage of the scintillationdetector is that the number of emitted photons produced by thescintillation plate or crystal, upon interaction with ionizingradiation, is approximately proportional to the energy of the incidentradiation.

Further, for imaging applications, the scintillation counter isposition-sensitive, and can yield good quality, medium resolutionimages. However, the resolution is limited by several factors, includingthe plate or crystal thickness, the photocathode spatial resolution, andthe spatial separation between the active region of the scintillationplate and the photocathode surface. For the detection of higher energygamma radiation, higher atomic number (i.e., high-Z) materials arecommonly used (e.g., LSO, BGO, CsI, etc.), and/or thicker scintillationcrystals (e.g., 3 cm instead of 1 cm). In the case of thickerscintillation crystals, the increased thickness reduces imageresolution.

Based on the foregoing, there is a need for a radiation sensor with highresolution capability, fast pixel response, minimal dead-time, improvedradioisotope identification, and which can be manufactured in largesizes relatively inexpensively.

SUMMARY OF THE INVENTION

One embodiment of the present invention is an ionizing radiationdetector formed from a plasma panel that includes a front substrate, anda back substrate that forms a generally parallel gap with the frontsubstrate. X (column) and Y (row) electrodes are coupled by gasdischarge events to define one or more pixels. Impedances are coupled tothe X and Y electrodes, and a power supply is coupled to one or bothtypes of electrodes. Discharge event detectors are coupled to theimpedances.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a columnar-discharge plasma panelradiation sensor in accordance with one embodiment of the presentinvention.

FIG. 2 is a perspective view of a surface-discharge, plasma panelradiation sensor in accordance with one embodiment of the presentinvention.

FIG. 3 is a perspective diagram of a front substrate for a dual-level,single-electrode, micro-segmented, columnar-discharge plasma panelradiation sensor device in accordance with one embodiment of the presentinvention.

FIG. 4 is a perspective diagram of a front substrate for a dual-level,multi-electrode, micro-segmented, columnar-discharge plasma panelradiation sensor device in accordance with one embodiment of the presentinvention.

FIG. 5 is a block diagram of an example of a plasma panel radiationsensor tiled modular array system in accordance with one embodiment ofthe present invention.

FIG. 6 illustrates an example of a linked plasma panel radiation sensorlocal area network detection system in accordance with one embodiment ofthe present invention.

FIG. 7 is a block diagram illustrating the circuitry attached to twocurrent-directional electrodes (X and Y) defining a pixel centroid of aplasma panel radiation sensor in accordance with one embodiment of thepresent invention for electrically sensing a discharge site.

FIG. 8 is a block diagram illustrating detection event processingcircuitry that is attached to the circuitry of FIG. 7 in accordance withone embodiment of the present invention.

FIG. 9 is a perspective view of two vertically-stacked plasma panelradiation sensor configurations in accordance with two embodiments ofthe present invention.

FIG. 10 illustrates a picture-frame type, spacer-seal arrangement inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION

One embodiment of the present invention is the use of a gas dischargepanel (referred to as a plasma display panel (“PDP”) if originallydesigned for display applications, or plasma panel (“PP”) if designedfor photon radiation detection) coupled to electronic circuitry to forma plasma panel sensor (“PPS”) for detecting radiation.

The PPS in accordance with one embodiment of the present invention canbe fabricated with a high resolution pixel structure and in large sizesat very low cost by leveraging into the multi-billion dollarmanufacturing infrastructure now in place for the high-growth PDPsegment of the consumer electronics industry. Operationally, oneembodiment of the present invention is a direct, high-gain, position andintensity sensitive, digital counter/detector of electrons emitted as aconsequence of the interaction of ionizing photons (i.e., gamma-rays orX-rays) and/or particles (e.g., fast or slow neutrons) with appropriatematerials that can internally convert such radiation into“free-electrons”, without having to go through a scintillationconversion step to visible or ultraviolet photons and subsequentdetection by an expensive photo-sensing element.

In general, a single, solitary electron (i.e., free-electron) enteringthe high-field, high-resolution, pixel space of a suitably designed,plasma panel cell in accordance with an embodiment of the presentinvention can experience almost instantaneous internal electronamplification with a gain of approximately 11 orders-of-magnitude (i.e.,10¹¹), without external amplification, and without loss of spatialresolution. The resulting electron avalanche, which is both confined andself-contained by the high field that defines each pixel's cell space,occurs within a couple of hundred nanoseconds. The PPS, in accordancewith an embodiment of the present invention, does not employscintillation materials, nor require the use of high-pressure,expensive, high-purity isotopic gases such as ³He or ¹⁰BF3, althoughsuch gases might be useful for certain configurations/applications.

In a typical alternating current PDP (“AC-PDP”) designed for videodisplay applications, each “OFF” cell sits at a voltage significantlybelow the threshold potential needed to initiate a plasma dischargebetween opposing electrodes. These electrodes are typically covered witha thick-film dielectric for charge storage. The thick-film dielectric isin turn coated on its top surface with a thin-film, secondary-electronemissive, dielectric material (e.g., MgO) having a low work-functionthat can readily supply electrons to the low voltage discharge gas(typically a neon-xenon mixture) and thereby sustain the plasmadischarge which is an electron-amplification avalanche event. Withregard to direct current PDP (“DC-PDP”) devices, the plasma discharge istypically current-limited by an external series resistor, as there is nodielectric layer (over the discharging electrodes) to store an opposingcharge to limit the discharge current. DC-PDPs, for a variety oftechnical reasons, were never commercialized for video/TV-typeapplications, but serve a few specialty niche markets, primarily for lowresolution, monochrome, alphanumeric applications. In conventionalAC-PDP or DC-PDP devices, mechanistically, for the selected dischargecell, the address voltage “boost” that is provided on top of the basevoltage, yields a composite resultant cell voltage in excess of the“firing voltage” needed to initiate a gas-discharge. Such gas-discharge“firing” events, however, only reliably occur if an adequate supply of“priming” electrons are present to help initiate a cell discharge,otherwise a much higher voltage is required to initiate gas breakdown.This latter situation, of being able to set the base cell voltagesignificantly above the so-called firing voltage due to the absence ofpriming electrons, is the desired condition under which embodiments ofthe present invention can work best, and is therefore a design goal fordevice operation with maximum sensitivity.

For a plasma panel to successfully function as a radiation sensor inaccordance with an embodiment of the present invention, it is configuredsuch that the “priming” electrons needed to initiate the discharge comefrom an external source of ionizing radiation (e.g., gamma-rays,neutrons, etc.). This means that the plasma panel should be structurallyand electronically designed with a very different set of design rules asthose governing its use as a display device. Thus, to maximizesensitivity, the panel should be designed to eliminate or at leastminimize all possible internal sources of priming electrons. Thegreatest reservoir of such electrons in the panel that are “potentiallyavailable” to become internal priming electrons, is the stored chargecontained on the dielectric surfaces (“walls”) interfacing the gas. Atany given ambient temperature there will always be a dynamic equilibriuminvolving gas-phase collisions with the dielectric surface resulting incharges/electrons leaving the surface and entering the gas, and therebybecoming priming electrons. Elimination of pixel wall charge is perhapsthe single most critical factor with regard to successfully configuringand operating a plasma panel device (AC or DC) as a PPS radiationdetector in accordance with an embodiment of the present invention. Itis thus necessary to minimize, if not eliminate, as much of the internalpanel dielectric structure and materials as possible. This meanselimination of internal dielectric barrier structures including wallsand ribs of various geometries, as well as the elimination of allphosphor dielectric particles. It therefore also follows that monochromeplasma panel structures configured for direct luminescence from the gasare inherently better suited for use as PPS-devices in accordance withembodiments of the present invention than color panels configured fordirect luminescence from a phosphor.

AC-PPS Versus DC-PPS Structure

Embodiments of the PPS in accordance with the present invention may beAC (“AC-PPS”) or DC (“DC-PPS”). In comparing AC-PPS to DC-PPSconfigurations, the latter has a device structure and mode of electronicoperation much more akin to that of a GM-tube than the former. Inparticular, like the GM-tube, the DC-PPS in its simplest embodiment of a“columnar-discharge” configuration (as shown in FIG. 1, discussedbelow), is structured with its “bare” cathode facing a “bare” anode(with no dielectric layer in-between), separated by a gap and filledwith a discharge gas. In this structure, the DC-PPS electrodes, likethose of the GM-tube, are normally kept at a constant“ready-to-discharge” voltage via direct connection to a steady,well-regulated, DC power supply circuit. Such is not the case with anAC-PPS, which is constantly cycling back and forth between twoessentially opposite voltage plateau settings. For the transition periodduring which the voltage is changing, the device should be unresponsiveand radiation-generated free-electrons will be “lost” (i.e., notcounted). In addition, a “conventionally” structured AC-PPS device, withdielectric over the discharge electrodes, will require complexdrive-waveforms incorporating dielectric wall-charge erase functions(for neutralizing accumulated charge stored from previously lit “on”pixels), which at best can be only partially effective. However, if thestored wall-charge from the top dielectric layer can be efficientlyerased, then the AC-PPS structure could offer some important benefits.For example, the AC-PPS top-layer MgO refractory coating could provide amore stable and efficient electron emissive surface than the metallicDC-PPS electrode material. In addition, the AC-PPS emissive thin-filmovercoat could be improved upon regarding interaction with ionizingradiation (i.e., absorption and/or inelastic scattering), by replacingthe “standard”, low-Z, MgO coating with a higher-Z, electron emissiveoxide such as La₂O₃, Eu₂O₃, etc. However, all such candidatesecondary-electron emitter materials (like MgO) must be chemically andthermally stable, sputter-resistant, and thermally activated atprocess-compatible temperatures.

FIG. 1 is a perspective view of a columnar-discharge PPS radiationsensor 10 in accordance with one embodiment of the present invention.Sensor 10 includes a first (front) substrate 12 and a second (back)substrate 14, separated by a gas filled gap 18. Sensor 10 includescolumn (“X”) electrodes 16 and row (“Y”) electrodes 17. Sensor 10further includes a first conversion layer 19, a second conversion layer20, first and second dielectric layers 21 and 22, and a third and fourthconversion layer 23 and 24. Sensor 10 is a DC-PPS structure. With regardto the structure and materials to be employed on the back plate in FIG.1, the configuration and materials can mirror those chosen for the frontplate as shown. In this way, ionizing radiation (i.e., photons orparticles) that passes through the front plate has a chance ofinteracting with the back plate and having some fraction of generatedfree-electrons finding their way to the gas and initiating a detectionevent. However, for economic reasons, some embodiments of the inventionsdescribed herein associated with FIG. 1 may utilize a back plateconstruction that eliminates one or both of the conversion layers 23, 24and possibly the second dielectric layer 22 (if the second substrate isan insulator), or uses less exotic (i.e., expensive) materials thanthose employed on the front plate.

The columnar-discharge DC-type structure, as shown in FIG. 1, can bephysically converted into a conventional AC-type structure by theaddition of a dielectric layer on top of: (1) the front substrateelectrodes, or (2) the back substrate electrodes, or (3) both sets ofelectrodes. However, as discussed above, to maximize sensitivity,PPS-devices in accordance with embodiments of the present invention haveminimized internal priming and charge storage, which would normally bethought of as being much more difficult for AC than DC type devices,since charge storage is fundamental to AC-PDP operation. Nevertheless itis possible to minimize charge storage in an AC-PPS, precisely becauseit is not a display device and does not require a dielectric over the Xand Y discharge electrodes or a conventional AC-type sustain/drivewaveform. A successful AC-PPS device, like a DC-PPS, should undergo onlya single gas-discharge avalanche event, as opposed to a continuum ofsuch events. All of the PPS structures in accordance with embodiments ofthe present invention that are disclosed in FIG. 1 and other figuresbelow, can therefore be operated not only as DC devices, but also“unconventionally” as AC-type devices (i.e., without the top dielectriclayer) by driving them directly with a properly configured andwell-regulated AC power supply. For such operation, a highlysputter-resistant electrode system should be used (similar to that usedfor a DC-PPS). It is noted that the duty-cycle for a PPS will beorders-of-magnitude lower than for a PDP-video monitor, and so a “bare”metallic electrode AC-PPS structure could be quite reasonable.

Surface-Discharge PPS Structures

Plasma panel configurations are commonly described in terms of theiranode-cathode/gas-discharge shape. For example, if the anode and cathodeare located on opposite substrates and are orthogonal to one another,such as in FIG. 1, then the gas-discharge between the two plates at theintersection of the orthogonal electrodes assumes a “columnar” dischargevolume shape. Alternatively, if the anode and cathode are alongside eachother on the same substrate, then the gas-discharge will be confined tothe plate surface and assume a “surface” discharge arc shape. Both typesof discharge structures have their advantages with respect to a PPSradiation detector, and each of the two basic structures can have manyvariations. One advantage of the surface-discharge PPS device(“SD-PPS”), as shown in FIG. 2, is that in eliminating the dischargeelectrode from the first substrate (i.e., moving it to the secondsubstrate in FIG. 2), a degree of freedom is gained with respect tochoice of substrate materials, structural modifications and gas gap.With regard to the latter, in a columnar-discharge PPS (“CD-PPS”), thegas gap 18 and the X-Y discharge gap (also 18) are essentially one andthe same (see FIG. 1). However, in the SD-PPS (see FIG. 2) the gas gap32 and the X-Y discharge gap 33 are decoupled and essentiallyindependent of one another. In other words, the SD-PPS structure canpermit the use of extremely large gas gaps, on the order of those foundin a GM-tube, for enhanced incident photon/particle interaction andtherefore increased PPS collection efficacy, with minimal effect on thesurface-electrode field strength and X-Y gas discharge dynamics. Onesuch means to physically implement this capability is by adding suitablytall peripheral spacers 128 along the panel perimeter seal 126 (see FIG.10). For example, the gas “cross-section” media thickness for a GM-tubeis typically between 10-100 mm, whereas in a commercial PDP (e.g., atelevision set) the gas interaction cross-section is only about 0.1 mm.In an SD-PPS, the panel gas-gap 32 (FIG. 2) can be easily increased byat least one to two orders-of-magnitude, for example from 0.1 mm to 1.0mm, or even to 10 mm or more. In fact, with a “picture frame” type ofspacer-preform peripheral structure 128, 129 (FIG. 10, bottom rightinset), the gas gap could be made as large as in a GM-tube. By usinglarge gas gaps, the SD-PPS, in accordance with an embodiment of thepresent invention, could be modified for maximum gas mixture attenuationby employing similar gaseous components as those found in comparableGM-tubes, such as ³He based gas mixtures for use with a PPS designed forneutron detection. This possibility is only practical however for SD-PPStype configurations, where the discharge path length is independent ofthe gas gap and so no sacrifice in pixel/electrode field strength isincurred with the much larger gas gap.

One SD-PPS embodiment, as illustrated in FIG. 2, utilizes a simple,planar design for the first substrate 34 with a single conversion layer31 for converting incident photon (e.g., gamma-rays) or particle (e.g.,neutrons) ionizing radiation into free-electrons that can be emittedinto the gas. FIG. 2 is a perspective view of one embodiment of thesurface-discharge, plasma panel sensor 30, with first substrate 34,conversion layer 31, and a second substrate 36. Sensor 30 furtherincludes on the second substrate, X-electrodes 38, Y-electrodes 37 andan insulating dielectric 35. First and second substrates 34, 36 form agas gap 32 which is filled with a gas. The simple SD-PPS structure shownin FIG. 2 relies in part on the close pairing of adjacent X-Y (i.e.,column-row) electrodes for gas discharge pixel localization. As can beseen in FIG. 2, the discharge gap 33 for a given X-Y electrode pair isshown to be smaller than the pixel gap 39 between two adjacent pixelpairs. This gap difference is significant and the smaller discharge gap33, relative to the larger pixel gap 39, is meant to prevent dischargespreading and delocalization, which would amount to loss of imageresolution and detection sensitivity. However, the disadvantage ofhaving too large of a pixel gap (or isolation gap as it is sometimescalled) is that it reduces the PPS detector spatial resolution and canpossibly cause “dead spaces” to develop within the plasma panel,essentially reducing the effective collection area or activefill-factor, thereby reducing the efficacy of the PPS as a radiationdetector. Therefore, to further enhance discharge localization withoutreducing image resolution, additional gas discharge panel structuralisolation components can be employed. One such embodiment would useinternal spacer-barrier walls to achieve cell isolation between adjacentpixel rows and/or columns. Alternatively, either a relatively deepsurface depression/trench, or an isolation electrode, can be used toeliminate avalanche spreading by preventing the discharge from jumpingtransversely across from one X-Y electrode line pair to the next viaelectrical crosstalk. The use of an “isolating” electrode (with voltagebias) between, and parallel to, each “line pair” of adjacent X-Y surfaceelectrodes has the advantage of being easier to fabricate than aspacer-barrier wall.

A number of surface-discharge configuration variations are possibleconstituting a family of different SD-PPS embodiments, such as, forexample, the use of not one, but two or more conversion layers on thefirst substrate like those shown for the columnar-discharge structure inFIG. 1. Alternatively, the first substrate itself can function as theconversion layer, thereby eliminating the separate conversion layer 31shown in FIG. 2. In addition, by not having electrodes on the firstsubstrate, there is no need for a first substrate dielectric layer (asshown in FIG. 1) and to incur the associated problems of: (1) chargestorage by the dielectric layer, and (2) inefficient free-electrontransport from the conversion layer, through the dielectric layer, andinto the gas. In an embodiment utilizing the above properties toadvantage, the first substrate can be fabricated out of a conductiveconversion layer material (e.g., gadolinium). Similarly, the firstsubstrate is not limited to being a simple, flat, structure, as therecan be advantages to using a variety of 3-dimensional substrate shapessuch as the external rib structures discussed below and shown in FIGS. 3and 4. Such 3-dimensional structures, however, are not limited in anSD-PPS to the external substrate surface as they are in a CD-PPS. Thereare many advantages to using 3-dimensional configurations internally forperformance enhancement, sensitivity optimization and sourceradioisotope identification (see below). For example, the planarconversion layer 31 shown in FIG. 2 can be replaced by a multi-levelplateau structure such as the dual-level plateau configurations 49 and69 in FIGS. 3 and 4 respectively. By such substitution, the3-dimensional conversion layer surface would be directly interfacing thegas and thereby not have to suffer the inefficiencies and spatialresolution loss associated with having to first pass through adielectric insulation layer 21, 44, 64 and electrode layer 16, 42, 62(see FIG. 1, FIG. 3 and FIG. 4), before reaching the gas. The presenceof any intermediate media between the conversion layer and gas willcause some amount of attenuation loss of the conversion layer generatedfree-electrons from reaching the gas and thereby initiating a gasdischarge avalanche detection event. Finally, because the discharge gap33 is independent of the gas gap 32 in SD-PPS structures, suchstructures have much more tolerance for gas gap non-uniformity and aretherefore compatible with reducing and in many cases totally eliminatingthe need for internal panel spacers (e.g., see internal spacers 112 inFIG. 10).

As previously noted with regard to the CD-PPS structures, all of theSD-PPS structures in accordance with embodiments of the presentinvention as disclosed in FIG. 2 and in the discussions above and below,can be operated not only as DC devices, but also as either conventionalor unconventional AC-type devices (i.e., conventionally with adielectric layer over the surface electrodes, and unconventionallywithout such dielectric layer). However, the DC mode of operation wouldgenerally be the preferred mode for the reasons previously discussed. Ofthe two AC surface-discharge configurations, the “unconventional”structure reduces the problem of stored surface charge and so would beeasier to implement physically as well as electronically by simplydriving the X- and Y-electrodes with a properly configured andwell-regulated AC power supply. Further embodiments can employadditional conversion layers 23 and 24 on the second substrate toenhance efficiency by interacting with ionizing radiation that may havepassed through the first substrate without attenuation.

To further enhance efficiency, one embodiment of the present inventionis a double-surface-discharge PPS (“DSD-PPS”) radiation detector thatcan be operated as above in either the AC or DC mode. The basicadvantage of the DSD-PPS is in having relatively independent, activepixel structures on both the front and back substrates for moreefficient collection of liberated free-electrons. The SD-PPS structureshown in FIG. 2 can be modified to form a DSD-PPS structure byreconfiguring the first substrate structure to look and function likethe structure shown on the second substrate. In one such embodiment, thefirst substrate would include X1- and Y1-electrodes, with theY1-electrodes formed in two layers separated and insulated by adielectric layer as shown for the Y-electrodes in FIG. 2 for the secondsubstrate. Thus in one such embodiment of the DSD-PPS, the first andsecond substrate structures are essentially mirror images of oneanother, including both the X-electrode and more intricate Y-electrodeconfigurations. In this embodiment, the two sets of X- and Y-electrodeson the two substrates can operate relatively independent of one another,and as such the electrode orientations of the X1- and Y1-electrodes onthe first substrate can be either parallel or orthogonal to the X2- andY2-electrodes on the second substrate. Similarly, two different powersupply circuits are utilized in the DSD-PPS to provide the requiredvoltage potential difference between the X- and Y-electrodes. Withregard to relative electrode orientation, it is noted that orienting thetwo sets of first and second substrate X and Y surface-dischargeelectrodes orthogonal to one (i.e., X1 perpendicular to X2, and Y1perpendicular to Y2) another could enhance pixel discharge localizationand therefore enable higher spatial resolution sensors.

The PPS as a High Performance, Hybrid Solid State Radiation Sensor

For gas-filled radiation detectors in accordance with embodiments of thepresent invention, significant reaction to low-energy photon or neutronionizing radiation can occur both in the gas detection media and thedevice walls. However, even for device fill-gases with the highestattenuation cross-sections (e.g., Xe for gamma-rays or ³He forneutrons), very little interaction can occur in the gas if the deviceoperates at a reduced pressure (e.g., 0.5 atm) and has a small gas gap(e.g., 0.1 mm), as is the general case for the PPS, with the exceptionof the surface-discharge structures discussed above which can have largegas gaps. With higher-energy radiation, “absorption” will occur almostexclusively in the device walls, assuming that the walls are made ofappropriately high attenuation materials—e.g. high-Z materials in thecase of gamma-rays, and high interaction cross-section materials forneutrons. Yet only electrons that manage to escape the wall surface intothe gas (or are created in the gas) can be amplified and counted. Thusthe gas mixture in the PPS does not in general serve as the conversionmedia, but as a signal amplification media, and for this reason the PPSshould not be thought of as a gas detector in the conventional sense,but rather as a hybrid solid-state detector. To maximize the conversionof ionizing-photons or fast/slow neutrons to free-electrons,high-efficiency radiation “absorbing” conversion layers are incorporatedinto the basic plasma panel structure, such as layers 19, 20, 23 and 24of FIG. 1. With this addition, the PPS can act as a highly-integrated,macro-array of parallel pixel-sensor-elements capable of detecting asingle free-electron generated by incident ionizing photon and/orneutron radiation.

In the voltage domain that the PPS operates, most electrons escaping thepanel wall into the gas would be expected to “immediately” (i.e., within˜100 nanoseconds) undergo electron multiplication in the very highelectric field (i.e., ˜5×10⁶ V/m) of the nearest pixel. The result ofthis electron multiplication is a “plasma” gas discharge pulse (i.e.,avalanche) that can be “seen” as an activated light-emitting pixel. Inall plasma panel devices (whether DC or AC), this process is designed tobe self-limiting by virtue of an opposing impedance in series with thepixel that cuts off the discharge by dropping the pixel voltage beforetoo high a current develops. The output pulse from each pixel willtherefore always be about the same regardless of the number of“free-electrons” initially entering the same local pixel space at thesame moment in time. However, other free-electrons created by the sameionizing radiation, entering a different pixel field space, even at thesame time, can create a different discharge and be counted separately.Thus, the number of pixel discharges should reasonably reflect thenumber of free-electrons created by the incident ionizing radiation.This is the reason that plasma panels, when structured in accordancewith embodiments of the present invention, have the capability of actingas intensity-sensitive, digital-counting devices. In this respectPPS-devices can respond to the energy of ionizing-photons in much thesame manner as a scintillation counter. However, GM-tubes have no suchcapability because the entire tube acts as a single pixel and thereforecannot count “simultaneous” electrons ejected spatially from differentregions of the cathode wall surface. In essence, given the very highspatial resolution capability of the plasma panel electrode matrix(˜0.01 mm), and the small gas gap (e.g., 0.1 mm or less), and fastdischarge time (i.e., a few hundred nanoseconds), it is most improbablethat multiple free-electrons will enter the same PPS pixel space at thesame time, and thereby not be detected. For these reasons, PPS devicesin accordance with embodiments of the present invention can be intensitysensitive to incident radiation in a somewhat analogous manner asproportional counters, but with potentially higher spatial resolutionand frame update rates (i.e., ˜1000 fps) than scintillation-basedradiation detectors. The latter, which typically employ PM-tubes, a-Si,CCD or CMOS photon sensors, are much more expensive per unit area thanthe PPS in accordance with embodiments of the present invention.

PPS Response to Incident Radiation

The spectral energy range of greatest interest for detectingionizing-photon radiation (i.e., gamma-rays and/or X-rays) for mostapplications, generally falls between the energies of about 20 keV to2,000 keV (i.e., 0.02 to 2 MeV). In this range, incident photonattenuation will occur primarily via a combination of photoelectric andCompton effects. Of the two processes, the basis for the majority ofionizing radiation detectors at photon energies up to a few hundred keVis the photoelectric effect, which causes the initial production of asingle free-electron and a corresponding positive atomic ion. Thisregion can be extended up to around one-half MeV in energy by the use ofhigh-Z sensor materials. However, at higher photon energies, Comptonscattering generally becomes the dominate mechanism. The greater theenergy of the incident photon (i.e., up to several MeV), the greater thedegree of Compton scattering and hence the broader the required spectralresponse of the PPS. Yet regardless of mechanism, only those electronsthat manage to escape from the device wall into the gas can be amplifiedand thereby sensed. To take advantage of this conversion effect ofionizing-photon radiation to free-electrons in the device walls, ahigh-Z, high-density conversion layer(s) is incorporated into the PPSstructure, such as conversion layers 19, 20, 23 and 24 of FIG. 1. Yeteven without such a high-Z conversion layer, embodiments of the presentinvention, when tested, were able to be successfully modified tofunction as ionizing-photon sensors and demonstrated good sensitivity tothe following three gamma-ray sources (which were the only radioisotopestested): ⁵⁷Co (122 keV), ⁹⁹Tc (143 keV) and ¹³⁷Cs (662 keV). From amaterials design viewpoint, the positive detection results wereespecially impressive because of the non-optimum materials construction,i.e., the plasma panel walls were of relatively low-Z materials and thethicknesses were wrong for realizing high gamma-ray sensitivity. Atleast an order-of-magnitude improvement would be expected in plasmapanels designed and built from the outset to be PPS radiation detectors(and not display panels) with optimized conversion-layer wall materials,as well as higher E-fields, an improved Penning gas mixture and betterelimination of wall charge. In contrast to gamma radiation, the responseof a PPS to slow-neutrons can be either via a single-step or multi-step,particle/photon, reaction process. Yet regardless of mechanism, thefirst step usually involves (n,α), (n,p), (n,γ) or (n,e_(ic)) typereactions; it is noted that the latter reaction constitutes directemission of an internal conversion electron (e_(ic)) upon absorption ofa slow-neutron. If the initial step is (n,α) or (n,p), then a secondconversion step will follow and most likely involve electron (e) and/orgamma-ray emission. In either case, there will almost certainly be (γ,e)tertiary reactions. In summary, incident slow-neutrons can eitherdirectly or indirectly generate free-electrons and gamma-rays, which inturn can be detected by the PPS in accordance with embodiments of thepresent invention. Thus the efficiency and sensitivity of the PPS tothermal-neutrons at the “front-end” of the detector (e.g., firstconversion layer 19 of FIG. 1), will need to be matched by theefficiency and sensitivity of the PPS to gamma-rays and free-electronsat the “back-end” of the detector near the dielectric-gas interface 21(e.g., the second conversion layer 20).

The PPS in accordance with embodiments of the present invention can alsobe designed as a fast-neutron detector by modifying the conversionlayer(s) material(s) and dimensions. As with conventionally designedneutron detectors, fast-neutrons are more difficult to detect thanslow-neutrons, and so PPS sensitivity to fast-neutrons would be expectedto be less than that for slow-neutrons. In addition, the choice ofsuitable solid-state conversion materials for fast-neutrons is morelimited than for slow-neutrons, and usually involves incorporatingmaterials containing ¹H and/or ⁶Li. In the case of the former, saturatedhydrocarbon polymers have conventionally been employed such ashigh-density polyethylene (“HDPE”). In the case of the latter, twofrequently chosen materials include ⁶Li₂O—SiO₂ based inorganic glassesand thick-film or thin-film deposited ⁶LiF coatings. Although the PPSefficiency for fast-neutrons should be less than for slow-neutrons, thebasic device advantages would still be retained, such as: low cost forlarge collection area, high spatial resolution, high internalamplification, high pixel imaging capability, source isotope ID, ambientoperation with high signal-to-noise ratio, high fill factor, etc. Mostimportantly, the novel PPS device structure in accordance withembodiments of the present invention facilitates use of new materialpossibilities that have the potential to significantly enhancefast-neutron efficiency.

PPS Sensitivity, Detection Capability & Efficiency as a Photon Detector

Experimental results of embodiments of the present invention can besummarized as follows: using less than 3 square inches of active sensor,a modified-PDP was able to detect weak radioisotope emission (i.e., 3.0μrem/hr) at a distance of 5 feet, or equivalently at 10 feet if two suchdetectors were located on opposite sides of a 10 ft portal opening. Inother words, for cargo/vehicles moving through a 10 ft wide doorwayportal, two small (i.e., 2-3 inch) PPS detectors should be able toquickly capture an image of almost any ionizing-photon source emittingradiation at or below the background level. As previously stated, forthe PPS gamma-ray detector, at least an order-of-magnitude improvementin sensitivity is expected. However, because the technology is soinexpensive, one would not normally design a passive portal based ononly a few square inches of PPS detector. Instead, one would takeadvantage of the low cost and use at least a few square meters of PPSdetector in order to capture high resolution source images, at fastspeeds (e.g., 60 mph), and with spectroscopic radioisotopeidentification.

In comparing the PPS detector in accordance with embodiments of thepresent invention to a gamma-camera scintillation counter in which thespatial pixel resolution of the latter imager can be on the order ofabout 3 mm, it is noted that AC-PDPs with a pixel resolution as high as0.11 mm were produced more than a decade ago. Thus PPS-detectors withorder-of-magnitude higher intrinsic pixel spatial resolution than thosefound on scintillation detectors used in nuclear medical imaging arequite feasible. However, to realize such high spatial resolutions in aPPS imaging system, the apparatus in one embodiment incorporates anappropriately matched, high-resolution collimator plate. In terms ofefficiency, there are no obvious limitations to the intrinsic PPS deviceefficiency. However, intrinsic device efficiency is not actually thecritical parameter for most radiation detector applications. Typically,the most important parameter is the system or global efficiency. This isbecause the only way to beat the 1/r² global efficiency losses, is forthe detector system to “cover” the largest possible surface area so asto collect the largest solid angle of emitted radiation. This can onlybe practically achieved if the detector is very low in cost. Theprojected PPS-device cost, in accordance with embodiments of the presentinvention, of just a few dollars per square inch, is at least anorder-of-magnitude less expensive than competing “conventional” systemsand therefore can be taken maximum advantage of by covering the largestpractical detection area to achieve correspondingly high systemefficiencies. Given the projected PPS low cost, high sensitivity, fastresponse time, flat form-factor and extreme ruggedness, severalintriguing security applications are feasible, including: covering thebottom of a helicopter with PPS-devices in order to “hover” over a shipat sea (or a building) to detect and image a suspected “hidden”radiation source, or to cover the side of a truck or van with suchdetectors that could then park alongside a suspected building for thepurpose of imaging a source hidden within the building.

Multi-Level PPS for Extended Spectral Range Optimization

For applications involving a variety of possible radioactive sources,such as homeland security, it is desirable that a radiation detector beable to respond with high sensitivity over a broad spectral range.However, such a situation leads to a conflicting choice of devicematerials, dimensions and optimization solutions. To address thismultifaceted need, one embodiment of the present invention is amulti-level device structure as shown in FIG. 3 that can exploit boththe photoelectric and Compton effects via a micro-segmented internal PPSconfiguration that maximizes flexibility to tailor performance solutionsto different spectral ranges. FIG. 3 is a perspective diagram of a frontsubstrate 40 for a dual-level, single-electrode, micro-segmented,columnar-discharge PPS device in accordance with one embodiment of thepresent invention. Front substrate 40 includes column X-electrodes 42, adielectric layer 44, and dual-level plateau conversion plate 49.

At low energies, the front substrate plate 12 and conversion layer 19 ofFIG. 1 should have minimal thickness in order to maximize both theincident gamma radiation through the substrate into the conversion layerand the subsequent transport probability of resulting single, low-energyelectrons (i.e., generated by the photoelectric effect) making itthrough the conversion layer to the gas. In an embodiment of FIG. 3,these two functions are combined as the low height plateau region of theconversion plate 49. In contrast, at high energies the substratethickness is not a problem, as the primary issue is to achievesufficient Compton scattering interaction of the incident gammaradiation with the conversion layer for efficient generation of Comptonrecoil electrons (i.e., free-electrons) and subsequent electrontransport through the conversion layer into the gas. This latterrequirement can be met in the thicker plateau region of the conversionplate 49 as represented in FIG. 3. In one embodiment of this invention,the multi-level PPS front substrate 40 of FIG. 3 is mated to the backsubstrate 14 of FIG. 1, thereby replacing the front substrate 12 of FIG.1, to provide an extended range PPS optimization solution. By combiningthis embodiment, as described immediately above and as conceptualized inFIG. 3, with a modular component approach, an overall system designbased on a tiled-array configuration, as shown in FIG. 5 below, can beimplemented to achieve optimum system sensitivity over the broadestpossible spectral range. An important benefit of the multi-level PPSconfiguration is that it can be used to great advantage for radioisotopesource identification as discussed below. In its most basic embodiment,the “multi-level” PPS can be configured as a dual-level,single-electrode, micro-segmented PPS, the front substrate of which isshown in FIG. 3.

The PPS front substrate 40 in FIG. 3 can be viewed as an embodiment ofthe PPS-device 10 shown in FIG. 1. The primary differentiating featureis the dual-level plateau conversion plate 49 of FIG. 3, which can beviewed in terms of FIG. 1 as simply representing a “patterned” frontsubstrate 12 combined with a first conversion layer 19, and with thesecond conversion layer 20 omitted. The preferred material for thedual-level plateau conversion plate would most probably be a high-Zmetal, such as a tungsten alloy, that could be conveniently cast orformed into the pattern shown in FIG. 3. The back substrate for thedual-level, single-electrode, micro-segmented PPS can be essentiallyidentical to that shown in FIG. 1 for the standard PPS structure, butwith the requirement that the back substrate materials be closelymatched to the front conversion plate in terms of linear coefficient ofthermal expansion. It is noted that front substrate 40 in FIG. 3 doesnot have to be a metal; it could also be a patterned ceramic (or glass)incorporating such high-Z materials as: WO₂, HfO₂, etc. If the frontsubstrate is a high-Z insulator, then dielectric layer 44 in FIG. 3 (and21 in FIG. 1) can be eliminated and the X-electrodes 42 in FIG. 3 (and16 in FIG. 1) can be deposited directly onto the back of the patterneddual-level conversion plate 49.

Of particular importance in constructing a multi-level PPS in accordancewith embodiments of the present invention is to utilize differentthicknesses of converter plate material (i.e., plateau's) for differentoptimization solutions over a broad range of incident photon/particleenergies. In principle, a variety of plateau thicknesses could beincorporated within a single multi-level PPS device; alternatively avariety of different dual-level PPS devices, each optimized for adifferent spectral range, could be tiled to form a single arraystructure. The latter choice is probably the more practical, and isdiscussed in more detail below.

Radioisotope Identification Via PPS Spectral Response Transform

The dual-level, micro-segmented, PPS front substrate of FIG. 3 can bedesigned to take additional advantage of the inherent high spatialresolution resulting from the internal device micro-discharge cellstructure. A trapezoidal cross-sectional shape is shown in FIG. 3(although other shapes could be used) for the dual-height plateaustructure making up the outer, top surface of the conversion plate foreach PPS module of a module embodiment as shown in FIG. 5 below. Asshown in FIG. 3, the discharge electrodes located on top of the flatinner dielectric surface are parallel to, and electrically isolatedfrom, the “outer-rib” or grooved structure, and are by design centeredalternately under either a thin or thick plateau layer of theouter-ribbed conversion plate. For a given ionizing-photon flux fallingupon the ribbed conversion plate, a series of uniquely definedgas-discharge intensity ratio differences should be observed (withineach PPS) between electrodes centered under a thin versus a thickconversion plate plateau. Each individual electrode ratio value iscalled an “alternate-electrode module intensity ratio” or AEMI-ratio.The matrix or family of AEMI-ratios thus corresponds to a uniquespectral response function that directly relates to the PPS structureand the incident spectral energy distribution of the sourceradioisotope(s). The ribbed conversion plate 49 of FIG. 3 can thus beconceptualized as somewhat analogous to a “grooved transmission grating”for generating a “free-electron spectral response transform” representedby the AEMI-ratio. The matrix of observed and measurable AEMI-ratio isthus a function of the incident gamma-ray distribution, and should inprinciple be calculable from the PPS device dimensions, materialattenuation coefficients, material electron emissivity (i.e.,electron-fraction associated with photoelectron andCompton-recoil-electron emission), material electron transport,conversion layer surface-work-function for electron ejection into thegas, etc. The above parameters are either known entities or can beestimated computationally (e.g., using Monte Carlo modeling tools) ormeasured experimentally. A given PPS module could be calibrated bygenerating an experimentally verifiable “module response look-up table”of AEMI-ratio values corresponding to various radioisotopes of interest.An unknown radioisotope would thus be identified by comparing the“measured sample” AEMI-ratio to that in a system look-up table residingin the detector processor 58 in FIG. 5 as discussed below. A mixture ofradioisotopes passing through the grooved conversion plate would thusgenerate a free-electron based spectrum of AEMI-ratio values requiringde-convolution algorithms in the detector processor to identify thesource radioisotopes. The above methodology is conceptually similar inmany ways to the de-convolution of a complex Fourier transform inconventional optical spectroscopy to identify and analyze unknown sourcematerials.

Achieving PPS optimization, including critical configuration, structuraland materials selection, in terms of various embodiments of the presentinvention, especially with regard to the above AEMI spectral responsetransform, should be theoretically guided but empirically based. Forexample, if a triple-electrode configuration under a suitably widerplateau were to improve device sensitivity and source radioisotopeidentification with regard to a modified AEMI-ratio, then atriple-electrode PPS configuration should be employed. By extension, theuse of a significantly wider plateau area on the conversion plate topsurface, in conjunction with a correspondingly wider multi-electrodepattern at the bottom surface (i.e., just below the plateau baseregion), provides an alternative configuration for source radioisotopeidentification, as shown in FIG. 4. FIG. 4 is a perspective diagram of afront substrate 60 for a dual-level, multi-electrode, micro-segmented,columnar-discharge PPS device in accordance with one embodiment of thepresent invention. Front substrate 60 includes multiple element columnX-electrodes 62, a dielectric layer 64, and dual-level plateauconversion plate 69.

The viability of the above modified configuration is based upon the factthat the greater the energy of an incident gamma-ray photon, the greaterthe degree of Compton scattering and hence the larger the number ofsubsequent photon-electron scattering events occurring in closeproximity to the original interaction site. With respect to theresulting multi-electrode discharge pattern at the bottom of thecorresponding wide-ribbed outer structure plateau (shown in FIG. 4), thenumber of neighboring/adjacent discharging cells should be directlyrelated to the initial photon scattering pattern within the conversionplate and thus the energy and identification of the source radioisotope.A modified AEMI-ratio comparison of the resulting Compton “cluster” or“balloon” scattering pattern associated with thick vs. thin plateauregions can be generated as described in the previous paragraph. Asabove, final source isotope identification would be made using anempirically-based, look-up table.

In summary, for the dual-height, micro-segmented PPS module, twoalternative configuration variations (i.e., FIGS. 3 and 4) of the newlyconceived AEMI-ratio method for source radioisotope identification aredescribed to eliminate the problem of “false-positives” from naturallyoccurring radioactive sources and/or other innocuous interferences(e.g., ⁴⁰K found in ceramic tiles, concrete, kitty litter, freshproduce, etc., or ⁹⁹Tc injected into persons undergoing diagnosticmedical nuclear imaging). It is worth noting that the drive voltage(s)for the PPS electrodes can also be dynamically adjusted to facilitate atotally different type of energy distribution spectrum for a givensource radioisotope. Also, the initial current response of each pixel asit builds up to avalanche should be representative of the particularenergy distribution of the source free-electrons as they leave theconversion layer and enter the gas. These additional methods could becombined to produce a more comprehensive “identification fingerprint” ofan unknown source should they be needed. Since each radioactive sourceshould have its own unique material fingerprint signature with regard tothe above-described methods, these could be programmed into thedetection processor 58, shown in FIG. 5 and compared to the unknownsource as it moves past the PPS in real-time.

PPS Modular Design Architecture A Tiled-Array System

To minimize system cost and maximize apparatus design flexibility andperformance, one embodiment of the present invention is a modular designbased on integrating a number of different, so-called “standard” PPSsensor modules, each optimized for detection within a specific spectralrange or for identification of a specific group of radioisotopes, toform a large area, “global” detector array system. More specifically, bycombining a multi-level device structure such as shown in FIGS. 3 and 4,with a modular component approach, an overall system based on a“tiled-array” configuration can be achieved for enhanced radioisotopeidentification with maximum detection sensitivity over the widestpossible spectral range. Conceptually, such detector arrays could beinterconnected with their electronics and control systems as shown inFIG. 5. FIG. 5 is a block diagram of an example of a PPS tiled modulararray system 50 in accordance with one embodiment of the presentinvention. System 50 includes detection circuitry 52, and a grouping ofPPS tiled module arrays 54 formed from multiple dual-level PPS modules56, the front substrates of which are shown in FIGS. 3 and 4. System 50further includes a local data collection & processing unit 51 composedof a sensor compiler 57 and detector processor 58. For such a system asshown in FIG. 5, the individual modular units making up each array canbe interconnected to provide improved, broad-band, high-resolution,source imaging and source radioisotope spectroscopic identification.With regard to the latter, the larger the array system collection area,the stronger the detection signal and hence the higher the systemspectroscopic energy resolution in terms of mixed source radioisotopeidentification (i.e., resolving overlapping spectroscopic linestructures from a radioisotope mixture). It is noted that the sourceimaging capability can be enhanced for any application by the use ofmultiple detector array units such as is commonly employed in scanningnuclear medical imaging or in optical astronomy, or in certain securitysystems such as the embodiment shown in FIG. 6.

If the various PPS tiled module arrays making up a detection system,such as the embodiment in FIG. 6, are separated by some distance, then a“pseudo-contiguous” detection signal can be extracted throughimplementation of an integrated aperture synthesis technique similar inconcept to the commercial firmware commonly used in digital cameras andvideo monitor scalars for pixel interpolation. More specifically, theso-called dead-spots between adjacent arrays can be handled withsoftware in a manner somewhat analogous to the way in which the humanbrain handles the “blind spot” at the center of the human eye's retina.

FIG. 6 illustrates an example of a linked PPS local area networkdetection system 70 in accordance with one embodiment of the presentinvention. System 70 includes a plurality of PPS detector modularhousings 74-76 that are coupled to a local data collection & processingunit 51, which is functionally the same unit as shown in FIG. 5. Eachhousing in FIG. 6 includes a PPS detector and electronics in accordancewith embodiments of the present invention, and an optional digital-videocamera. If such a camera is employed, then the PPS electronics can belocated along the module sides instead of at the module back, and thecamera augmentation 96 would be configured as described below for FIG.8. The digital output of housings 74-76 is sent to unit 51, and housings74-76 may be powered by a distributed power source 72. Through such asystem shown in FIG. 6, nuclear materials carried on vehicle 71 caneasily be detected.

The digital video camera shown below in FIG. 6, is one option for therecording of pixel discharge events in the PPS-detector. The more likelyoption would be to detect pixel discharge events directly through thedischarge electronic circuitry 52 shown in the top-left inset of FIG. 5,and discussed in more detail below. Note that array control, aperturesynthesis, etc. might be performed in a “central processor” located somedistance away. Interconnect could be via high speed USB, Firewire,Ethernet, etc. from the individual computer interfaces of each module inthe array. Each module would require adequate processing and memory toallow for buffering, protocol and transport of its respective dataframes or files to the computer system

System 50 of FIG. 5 incorporates the concept of what is referred to as“row-segmentation”, in which different rows can have different spectralsensitivity, but with all PPS modules within a given row beingidentical. For example, Row 1 in FIG. 5, consists of three identicaldual-level PPS modules (i.e., PPS-1 to PPS-3), each identicallyoptimized for response to incident radiation of low energy, for example20 to 100 keV. Similarly, Row 2 consists of three identical dual-levelPPS modules (i.e., PPS-4 to PPS-6), each optimized for incidentradiation medium energy, for example from 100 to 500 keV. Finally, Row 3consists of three identical dual-level PPS modules (i.e., PPS-7 toPPS-9), with each module optimized for the high energy region, forexample from 500 to 3,000 keV. The global array detection systemdepicted in FIG. 5, thus consists of 9 dual-level PPS modules, segmentedinto 3 spectral regions utilizing 3 distinct PPS optimization designs,which by way of example cover the region from 20 keV to 3 MeV. However,the number of modules, level of row segmentation, and number ofindividual dual-level PPS optimization designs, could easily be anorder-of-magnitude greater than that shown in the figures and discussedabove. It is noted that the term PPS “module” specifically refers to thecombination of plasma panel “sensor head” (e.g., FIG. 1-4) coupled tothe plasma panel head “local” electronic circuitry 52 in FIG. 5.

PPS Internal Structure & Efficiency as a Neutron Detector

The PPS in principle has the possibility of having a higher efficiencythan conventional slow-neutron imaging detectors, because the PPS hasthe capability of detecting thermal-neutrons via at least three (3)independent, parallel mechanisms operating simultaneously. The threemechanisms are: (a) direct detection in the gas of internal conversionelectrons generated in the conversion layer, (b) detection offree-electrons generated by charged particle interactions resulting fromincident neutron absorption in the conversion layer, and (c) detectionof free-electrons generated within the second conversion layer 20 ofFIG. 1 via a neutron-capture “prompt” gamma-ray reaction in the firstconversion layer 19 for an embodiment of the invention as shown inFIG. 1. Of these three mechanisms, the first one is a single-step (i.e.,direct electron) conversion and should therefore be of higher efficiencythan the multiple-step conversions.

An example of a materials structure based on FIG. 1 and designed to takeadvantage of the first mechanism (i.e., direct collection of internalconversion electrons), would be to use a coating of Gd metal for thefirst conversion layer 19, with a thin Gd oxide (Gd₂O₃) coating on topthat would serve the triple role of: second neutron conversion layer 20,secondary electron surface emitter to the gas, and electrode dielectricinsulator 21. The X-electrodes 16 might also be of Gd metal which couldprovide an efficiency advantage, or some other suitable metal such asNi, or even a metal alloy such as Gd—Ni. The bottom substrate could be amaterials mirror image of the top, but with a suitably thin and“transparent” Gd metal coating (or transparent tin-oxide coating) as thefourth conversion layer 24. It is noted that in the embodiment justdescribed, the conversion layers 20 and 23 would also serve as thedielectric insulator layers 21 and 22 respectively, thereby eliminatinga layer from each substrate. Additionally, by the conversion layersclosest to the gas (i.e., 20 and 23) being thin, and the conversionlayers closest to the substrate (i.e., 19 and 24) being metallic, thelatter would be designed to remove any stored charge accumulated by theformer. Finally it is noted that the advantages of using Gd in the aboveembodiment are significant, as it has the highest thermal-neutroncross-section of any element, is a highly efficient source of “internalconversion” electrons upon slow-neutron absorption, and is an efficientemitter of low energy, gamma-rays upon slow-neutron capture (i.e., thestrongest gamma-ray transition for ¹⁵⁷Gd occurs at 182 keV, and itsconversion electron at 72 keV is emitted in ˜39% of its slow-neutroncapture reactions).

In spite of the ability to detect neutrons via several parallel andsimultaneous mechanisms, the structure of the PPS in FIG. 1 is bothsimple and elegant (i.e., inexpensive). However, depending uponmaterials and efficiency optimization, the final structure inembodiments of the present invention could be a modified version ofFIG. 1. For example, it might not be necessary to have two conversionlayers, as the first and fourth conversion layers 19, 24 could beincorporated into a metallic substrate 12, 14 that could for examplecould be Gd metal or a Gd alloy such as Gd—Ni. If such were the case,then a thin dielectric such as Gd₂O₃ would be required for electrodeisolation and could serve a dual role as combined conversion layer 20,23 and dielectric layer 21, 22. As described above, the electrodes 16,17 could be Gd or Gd—Ni or some other metal. In addition to the aboveproperties, the Gd—Ni/Gd₂O₃/Gd—N system would have the double advantageof serving as a combined gamma-ray and slow-neutron detector.Alternatively, if the substrate was an efficient neutron absorbing andelectron emitting glass with some slight conductivity (e.g., from ⁶Li₂O)to eliminate wall charge, then the electrodes could be placed directlyon top, facing the gas. Finally, PPS devices structured as neutrondetector modules (e.g., Row 1 in the tiled modular array 50 of FIG. 5)could be integrated with PPS modules structured as gamma-ray detectors(e.g., Rows 2, 3, etc.) into a single global array system (such as shownin FIG. 5) capable of detecting both slow and fast neutrons as well asphoton radiation over a broad energy spectrum.

As previously discussed, embodiments of the present invention can bedesigned as a fast-neutron detector. Unfortunately, fast-moving neutralparticles normally have much greater penetration through a givenmaterial medium than slow-moving neutral particles. Hence, fast-neutronsare inherently less likely to react in such a manner as to create adetectable event, and so such detectors typically operate atsignificantly lower efficiency than slow-neutron detectors. However, asdiscussed below, the novel PPS device structure of the present inventionoffers a number of interesting possibilities for maximizing fast-neutrondetection efficiency beyond what might be typically achieved usingconventional fast-neutron detectors, and at lower cost.

Conversion of Slow-Neutrons to Free-Electrons

As discussed above, the conversion of incident slow-neutrons tofree-electrons can be either a single-step or multi-step process. Theefficiency of this process for a given device structure, such as thatshown in FIG. 1, will depend primarily on the choice of materials andthe material thickness dimensions. For example, since the initialcritical interaction is neutron absorption, materials having highneutron absorption cross-sections are obviously desirable, but thematerial branching ratio's and Q-values are also of great import. Amaterial may be good at absorbing neutrons, but if the resultingproduct(s) are not useful with regard to the detection mechanism, thenthe absorbed neutron will simply be “lost” and not counted. To beefficient in terms of both neutron conversion and subsequent eventdetection, the candidate material upon neutron absorption needs to emit“internal conversion” electrons, and/or useful gamma radiation, and/orenergetic particles that in turn can generate electrons and/or usefulgamma radiation, etc. By invoking the term “useful” gamma radiation,what is meant is gamma radiation of high enough energy that it can reachthe inner conversion layer close to the gas, so that the resultingfree-electrons are able to make it through the remaining conversionlayer thickness and enter the gas and cause an avalanche and be counted.If however, the emitted gamma radiation is too high in energy, it willhave a low probability of further interacting with the detector and canpass right through without being counted.

Candidate materials for the conversion layer can include the followingisotopes: ⁶Li, ¹⁰B, ¹⁴⁹Sm, ¹⁵⁵Gd and ¹⁵⁷Gd. Some of these materials,such as B and Gd, might be applied as a thin coating in their purestate, while others could be in the form of compounds such as BN and/orLi₃N. Essentially all of these materials need to be considered in theirpure oxide form (i.e., Li₂O, Gd₂O₃, Sm₂O₃, etc.), except perhaps boron,which probably needs to be combined with other oxides to create a stableglass. It is noted that a combination glass system incorporating, forexample, the oxides ⁶Li₂O and/or ¹⁰B₂O₃ either individually or together,such as in a lithium-borate, lithium-borosilicate orlithium-aluminoborosilicate glass (perhaps even enhanced with Gd₂O₃,etc.), might serve the dual function as both substrate and conversionplate. As an added benefit, the above oxides tend to have lowwork-functions with respect to secondary electron emission and so areelectron emissive. It is noted that for a variety of reasons, oneembodiment of the present invention would include the Gd/Gd₂O₃ systemfor the reasons provided above.

As previously discussed in conjunction with Surface-Discharge PPSStructures, it was disclosed that one advantage of the SD-PPS is thecapability of employing large gas gaps without adversely affecting thelocal gas discharge pixel field. In particular, one such embodiment wasmentioned in which ³He based gas mixtures could be used for neutrondetection. However, both ³He and BF₃ gas mixtures are commonly used asdetection media for slow-neutron radiation detectors and as such couldbe employed in an embodiment of the SD-PPS structure 30 shown in FIG. 2,or alternatively the DSD-PPS variant of that embodiment could beemployed as previously described.

Conversion of Fast-Neutrons to Free-Electrons

Essentially any slow-neutron detector can be “made” into a fast-neutrondetector by simply surrounding it with a suitable moderating medium toslow down and reduce the energy of fast-moving incident neutrons,thereby converting them into slow-neutrons. Unfortunately, thisbrute-force method suffers from three major problems. First, themoderating process by its very nature destroys the incident particleenergy information, thus eliminating all knowledge related to the sourceenergy spectrum needed for source identification. Second, the process inits entirety tends to be slow (e.g., hundreds of microseconds) due tothe time lag associated with multiple collisions occurring within themoderator in order to “thermalize” the fast neutrons, followed by slowdiffusion of the resulting thermal-neutrons from the moderator to theactive detector. Finally, the moderating media introduces efficiencylosses due to a combination of moderator neutron parasitic capturereactions and moderator thickness (i.e., path length) making successfuldiffusion of thermalized-neutrons to the active detector site a moredifficult and less probable event. Because of the above problems, thepreferred detection method is direct fast-neutron detection without theadded moderation step. Direct detection should to a large extent greatlydiminish or even eliminate the above-described problems. Basically twomechanisms for direct detection can be incorporated into a PPS-devicebased on the structure shown in FIG. 1. In the first method for directdetection, a material layer/coating is employed containing an isotopesuch as ⁶Li, which can directly interact with fast-neutrons via anα-particle nuclear emission reaction. The emitted α-particle can thensubsequently react within the conversion layer to generate free-electron(e) and/or gamma-ray emission, which in the case of the later caneffectuate a tertiary reaction resulting in electron emission. Thusregardless of the particular mechanistic path, the ⁶Li (n,α) reactioncan directly or indirectly (i.e., via a gamma-ray) lead to free-electronemission near the PPS gas interface, which in turn can result in a pixelavalanche detection/counting event.

In the second method, a material layer/coating is employed which caninteract directly with fast-neutrons via the mechanism of elasticscattering by light nuclei (e.g., ¹H) resulting in a recoil nucleuswhich in the case of ¹H is a recoil proton. For single scattering fromhydrogen, the incident neutron can transfer all of its energy to therecoil proton, although on average about one-half of the incidentneutron energy will typically be transferred. Because of the protoncharge, the recoil proton range will be relatively small, with secondaryemissions occurring similar to those resulting from the previouslydescribed ⁶Li (n,α) reaction. From a materials compatibility point ofview, ⁶LiH should prove to be an excellent choice for a PPS fast-neutronconversion layer coating, for the two individual isotopes (i.e., ⁶Li and¹H) are each ideally suited in terms of providing optimum response withrespect to the two, direct, fast-neutron, detection mechanisms discussedabove.

In terms of an embodiment of the invention based on FIG. 1 using a ⁶LiHfirst conversion layer 19, it is noted that LiH forms a stable compoundand should be compatible next to a second conversion layer 20 ofgadolinium, which is also capable of forming a hydride. The use of a Gdcontaining layer for neutron detection was previously discussed. In thisregard, introduction of a thermal/anneal fabrication-process cycle toenhance interstitial diffusion of ¹H (and also ⁶Li) from the outer LiHfirst conversion layer 19 to the inner Gd second conversion layer 20,should promote more efficient transfer of α-particles, recoil-protons,gamma-rays and free-electrons across the boundary between the twoconversion layers and towards the gas detection region, therebysignificantly increasing the efficiency of the PPS device as a directfast-neutron detector. It is noted that this expected increase inefficiency from interstitial diffusion should in part be the result ofreduced lattice mismatch (i.e. surface defect traps) between the twolayers.

In addition to the use of 6LiH for the first conversion layer asdiscussed immediately above, other possible first conversion layermaterials suitable for direct fast-neutron detection include: ⁶LiF whichcould form an inter-diffusion layer of stable gadolinium fluoride (i.e.,GdF₃), and ⁶Li₂O which similarly could form an inter-diffusion layer ofstable gadolinium oxide Gd₂O₃. In the latter case, a number of⁶Li₂O-based glasses are commercially available and might be used insteadof a pure ⁶Li₂O coating, although a pure ⁶Li₂O layer might offer higherefficiency performance and possibly lower overall cost. In summary, theembodiment of the fast-neutron detector structure as described above andshown in FIG. 1, utilizes a first conversion layer 19 that could be ⁶LiH(or ⁶LiF or ⁶Li₂O), and a second conversion layer 20 that could be Gdmetal. The thin insulating dielectric layer 21 could be Gd₂O₃, which ifthin enough could be partially conducting to facilitate the bleedingaway of stored charge. In another embodiment, the second conversionlayer 20 of Gd metal could be eliminated by going directly from thefirst conversion layer 19 to the insulating Gd₂O₃ layer 21. In eitherembodiment, the electrode structure 16 could be fabricated from avariety of metals, although Gd or a Gd alloy such as Gd—Ni would beobvious choices. In terms of the choice of substrates 12, the aboveembodiments can employ any of four categories of substrate materials: anon-reactive glass or metal substrate, a reactive ⁶Li-alloy metallicsubstrate, or a reactive ⁶Li₂O-based glass substrate. With regard to thestructure and materials to be employed for the back plate in FIG. 1, theconfiguration and materials can mirror those chosen for the front plateas shown, or can utilize a back plate construction that eliminates oneor both of the conversion layers 23, 24 and possibly the seconddielectric layer 22 (if the second substrate is an insulator), or usesless exotic (i.e., expensive) materials than those employed on the frontplate.

In terms of the two types of reactive substrates described immediatelyabove (i.e. the ⁶Li-alloy metal and the ⁶Li₂O-based glass), the desired⁶Li species can be introduced by means of either the “pure” isotope orby virtue of its 7.6% abundance in the naturally occurring element (note⁷Li constitutes the remaining 92.4%). Two possible candidate metallicsubstrates include alloys of the Mg—Li and Al—Li systems. However,higher levels of lithium can probably be incorporated into glasssubstrates as compared to metal substrates. Among possible lithiumcontaining, stable glass substrate systems are: borates, silicates,borosilicates, aluminosilicates, aluminoborosilicates, tungstates, etc.

As discussed above, by employing the method of direct fast-neutrondetection, the incident particle energy distribution information can bepreserved and used for purposes of source radioisotope identification.To accomplish this, a PPS front substrate configuration similar to thatshown in FIG. 3 or 4 can be used in the manner previously discussed.Given this structure, a molded material such as the above-described ⁶Licontaining metal or glass substrate might be the best choice for thegrooved dual-level plateau, outer conversion plate. In either case, thedielectric layers 44 and 64 shown in FIGS. 3 and 4 should also functionas a secondary conversion layer for the generation of free-electrons tothe gas from α-particles emitted by the outer plate ⁶Li (n,α) reaction.

As previously disclosed, the SD-PPS structure as shown in FIG. 2 has thecapability of employing large gas gaps without adversely affecting thelocal gas discharge pixel field. In taking advantage of this feature,one embodiment of the invention can employ ³He based gas mixtures as adetection media for fast-neutrons in either the SD-PPS structure 30 ofFIG. 2, or the DSD-PPS variant of that structure as previouslydescribed.

Gas System & Priming

As previously discussed, to maximize sensitivity plasma panels forradiation detection should minimize all internal sources of primingelectrons, and be configured such that the priming electrons needed toinitiate the gas discharge are generated from an external source ofionizing radiation such as gamma-rays or neutrons. Therefore, inembodiments of the present invention, the gas mixture, discharge gap,pressure and drive voltages, in combination need to inhibit all internalsources of priming electrons, while at the same time maximizingsensitivity to radiation-induced, free-electrons. The design elementsand methodology disclosed below are to inhibit unwanted priming,including minimizing gas phase metastables as well as the lifetimes ofgaseous excited state species and propagation of VUV emitted photons(via use of appropriate gas-phase quenching and VUV absorbingmolecules).

Penning Mixture—Addition of suitable, low ionization potential gaseouscomponent(s) at low concentration (i.e., molecular species such as O₂ orNO, or gas phase atoms such as Hg) to depopulate excited statemetastable species. The Penning mixture maintains high amplification andhence good avalanche initiation.

Discharge Spreading Inhibitor—Elimination of adjacent cell priming tomaintain spatial integrity of the initial localized discharge site (forhigh image resolution), thereby preventing the discharge from spreadinginto neighboring cells. The method is based on reducing the mean freepath of high mobility gaseous electrons or ions, by the addition of asmall amount of gas having both a large electron capture cross-sectionand a high propensity to form negative ions. Fortuitously, O₂ is such amolecule which can also form a Penning mixture (see above) with xenon todepopulate excited state metastable species. In addition, O₂ will absorbVUV photons as well as “stray electrons”, thereby further localizing thepixel discharge. In essence, the addition of O₂ (and possibly otherspecies such as NO) demonstrates that the discharge can be controlledand suppressed enough to prevent it from spreading to adjacent cells,but not so much as to prevent localized (i.e., single pixel) avalancheamplification.

Gas Collisional Cross-Section and Discharge Voltage—Panel gas mixturesat maximum pressure and with maximum average atomic-weight can be usedto increase the likelihood of electron capture/interaction and theprobability of secondary electron generation. For example, the basicxenon-in-neon gas mixture used in commercial PDPs was successfullymodified to increase electron collisional cross-sections by replacementof neon with higher atomic-weight gaseous components. As a side benefit,increasing the average atomic-weight can: (1) raise the voltage therebyincreasing the local pixel field, (2) shorten the discharge time thusallowing faster response and higher update rates, and (3) reduce devicedead-time.

“Typical” Gas Mixtures for Neutron Detectors—Neutron detectors such asthe microstrip gas chamber (MSGC) typically use ³He or ¹⁰BF₃ as theneutron converter and CF₄ as the quencher. In a columnar-discharge PPSdevice such as shown in FIG. 1, there would be very little benefit tousing ³He, because the panel operates at reduced pressure and has asmall gap (i.e. path length ˜0.1 mm). Additionally, helium has a verylow electron collisional cross-section compared to other inert gases,and so in this sense would actually reduce device efficiency. Finallythe use of CF₄ as a quencher would probably be extremely detrimental asboth carbon and fluorine ions in a plasma discharge environment would beextremely active chemically and thus destructive of the electronemissive surface (i.e., top dielectric facing gas and electrodes).However, in a SD-PPS configuration as shown in FIG. 2 (or in a DSD-PPS),the use of ³He for both fast- and slow-neutron detection could be ofsignificant benefit as the surface-discharge PPS structure canaccommodate much larger gas gaps.

Oxygen as “Penning” Charge-Transfer Dopant & Avalanche Control Agent—Thevisible emission spectra from two lighted, AC-PDPs (i.e., undergoingdischarge), filled with 100% Xe, and 99% Xe/1% O₂, have been recordedand analyzed. In the 100% Xe panel, the typical xenon “blue” emissionlines from Xe—I were observed with the two strongest lines in the xenonvisible region readily apparent at 467 and 462 nm. However, in the 1% O₂mixture, even though the gas mixture still contains 99% xenon, theprominent “blue” Xe—I lines were “missing”, but the strong “green” linesfrom 533 nm to 558 nm of ionized atomic oxygen were present anddominated the visible spectrum. The first ionization constants formolecular oxygen and atomic xenon are respectively at 12.07 and 12.13eV. Thus molecular oxygen and atomic xenon are coincidentally almost inperfect resonance, with the ionization energy of O₂ being just belowthat of Xe, making O₂ an excellent component for transferring the chargefrom ionized Xe to O₂. The fact that the emission spectrum of the 99%Xe/1% O₂ gas mixture has the characteristic “green” color of oxygen,without any of the “blue” bands of xenon, shows that excited Xeefficiently transfers its energy to molecular oxygen, which upondissociation (in an active plasma) emits from its lower energy levelscentered around the 538 nm peak that characterizes atomic oxygen. Thus asmall amount of O₂ can quench ionized Xe and excited Xe metastables, andthereby confine the PPS discharge to the local pixel site closest towhere the initial conversion event of an ionizing photon or neutron to afree-electron takes place. The fact that O₂ is also a good VUV absorberand scavenger of “stray” electron charge, simply adds to itseffectiveness as both a discharge spreading inhibitor and avalanchecontrol agent.

Device Structure

A PPS detector in accordance with an embodiment of the presentinvention, however configured, needs to be hermetically sealed, gasprocessed and generally fabricated in a manner similar to PDP devices.Yet unlike commercial PDP devices, PPS radiation detectors need to avoidinternal 3-dimensional dielectric surfaces (e.g., phosphors, etc.) tominimize pixel wall charge. As discussed above, for maximum deviceefficiency the PPS structure should be designed with a slight amount ofinternal dielectric conductivity to be able to “bleed off” residual wallcharge left on the internal dielectric surface at and adjacent to thesite of cell discharge at the gas-electrode interface.

Depending upon electrode pitch, the PPS in accordance with embodimentsof the present invention is a highly integrated array of between ˜10² to10⁶ micro-detection cells per square inch, each of which has thecapability of acting as an independent, position and intensitysensitive, radiation sensor. From a materials, fabrication, andtheoretical viewpoint, “an ultrahigh” PPS pixel resolution of ˜0.01 mmis eminently feasible, much more so than for a commercial television PDPproduct. This is because the most likely PPS configurations (e.g., FIGS.1-4) all resemble DC-PDPs, as opposed to the AC-PDP structures used inessentially 100% of all commercial PDP-TV products. From a fabricationviewpoint, the electrode resolution in a DC-PDP can be much higher andmuch better controlled than in an AC-PDP device, because DC-PDPelectrodes are not encapsulated under a highly reactive and chemicallycorrosive thick-film dielectric, which tends to undercut and underminethe AC-PDP electrode-material linewidth. As an important collateralbenefit, minimizing the PPS electrode-width for purposes of enhancingimage resolution, should also raise the intrinsic firing voltage,thereby increasing the local-pixel, electric-field strength, and hencethe device sensitivity.

Gamma-Ray to Free-Electron Conversion Plate/Layer—In order to achieveefficient gamma-ray to free-electron conversion, high density, high-Zmaterials should be used having: (1) a high material attenuationcoefficient with respect to the incident photon radiation, (2) highelectron emissivity within the conversion media (i.e., highelectron-fraction associated with photoelectron andCompton-recoil-electron emission), (3) efficient electron transportthrough the conversion media to the gas interface, and (4) a low surfacework-function with regard to electron ejection out of the conversionmedia and into the gas. Depending upon the spectral region, the choiceof conversion media can include both high-Z metals (see below) and/orhigh-Z dielectrics. Examples of the latter include dielectricsincorporating such “heavy” components as WO₂, HfO₂, PbWO₄, Bi₂(WO₄)₃,PbO, Bi₂O₃, Ta₂O₅, Gd₂O₃, etc.

High-Z, Dual-Function, Front Substrate/Conversion Plate—At high photonenergies, certain high-Z materials could in principle improve PPS deviceefficiency by serving the combined role as both front substrate andconversion plate, thereby eliminating for such an embodiment the firstand second conversion layers 19, in FIG. 1. For such configurations,“workable” tungsten alloys (e.g., W—Ti, W—Zr, W—Ta, W—Re, W—Pt etc.)that could be conveniently cast or formed into multi-level structures,such as those shown in FIGS. 3 and 4, are good candidates along withother high-Z metals, such as Ag—Cu, Pt—Ag, Pt—W, etc., which are highlyelectron emissive. With these materials, an appropriate thin-film oxidedeposited on the substrate bottom surface for electrode isolation wouldserve as a dielectric layer 21. Any of the standard electron beam orsputtered thin-film insulating oxides should work, assuming a good matchbetween the linear coefficients of thermal expansion. Materials such asWO₂, HfO₂, La₂O₃, Gd₂O₃, Bi₂O₃, etc., and combinations thereof (e.g.,La₂O₃—Gd₂O₃), need to be investigated because of their combination ofhigh-Z number and low work-function for secondary-electron emission.Candidate metals for the electrodes 16 should have high electronemissivity, and so silver, tungsten, and nickel are possible choices,perhaps flash-coated or alloyed with platinum for greater electronemissivity and resistance to sputtering. From a materials compatibilitypoint of view, an interesting combination for the substrate 12,14/dielectric 21, 22/electrode 16, 17 system embodiment might be:W-alloy/WO₂/W—Pt, other systems include: Gd-alloy/Gd₂O₃/Gd—Ni,Ta-alloy/Ta₂O₅/Ta—W, etc.

Partially Conductive Dielectric Layer—Use of a partially conductivedielectric layer positioned between the device electrodes and aconductive conversion layer in the various embodiments for FIGS. 1-4 canmake for an efficient PPS configuration because it provides a means tobleed away accumulated wall charge from the top insulating dielectriclayer that is in direct contact with the discharge gas. Three means offabricating such a partially conductive dielectric include: (1)selection of an appropriate dielectric material that is inherentlyslightly conductive, (2) fabricating a partially conductive dielectricby virtue of adding high mobility dopants such as lithium (e.g., in theform of Li₂O) or boron (which can be a semiconductor), (3) utilizing avery thin dielectric film that is partially conducting by virtue of itsextremely thin cross-section.

Pixel Response to Incident Radiation Flux (Intensity)—The disclosedembodiments of the PPS are for a true digital integrating device. Thelight intensity of an individual lit pixel is not particularlyimportant, only the fact that a pixel is either lit or unlit (i.e. “on”or “off”). The intensity integration is therefore based on the number oftimes that a particular pixel goes “on” in a fixed period. With thepossibility of a pixel lighting as many as 1,000,000 times per second(see below), the image gray scale capability of a PPS detector is mostimpressive and should be far better than for a PDP video monitor whichcan display 256 shades of gray in 1/60 sec, or equivalently turn “on”15,360 times per second. Besides individual and collective pixelintensity data, additional integration information to be collected bythe PPS module electronics would include the location of each pixel thatgoes “on”. For the described PPS device, the photon gain should be evenbetter defined in terms of uniformity than in a PDP video monitor,because the internal plasma panel structure is planar (i.e., very easyto fabricate) as opposed to three-dimensional (i.e., PDP devices requirehard-to-control, three-dimensional, process fabrication steps such asthe sintering and sandblasting of vertical barrier walls, and the thickfilm deposition of phosphor into a three-dimensional barrier structure).Given a pixel response rate potential of 10⁶ counts/sec (cps), and apotential pixel resolution of 10 μm corresponding to an image dotdensity of 6×10⁶ pixels/square inch, yields a PPS detector responsecapability of 6×10¹² Cps per square inch. This response capability isfar in excess of anything that might be experimentally encountered, sosignal saturation should not be an issue and PPS devices should thus becapable of providing linear digital intensity responses to essentiallyany source of incident radiation.

Photon Induced Charge Generation & Propagation—The mechanism of chargegeneration within the PPS depends upon the energy of the incidentphotons and the material composition of the conversion layer (i.e.,Z-number). For photon energies below about 200 keV, the device chargewill be generated primarily by photoelectric absorption of the incidentphoton resulting mostly in K-shell electron emission, with somesecondary X-ray photon and Auger electron emission. For higher energyincident photons (e.g., above 500 keV), the charge will be generatedprimarily by Compton scattering, resulting in creation of a successivenumber of recoil electrons. In terms of transport to the gaseous volume,since any incident photons (and neutrons, etc.) will, by definition,have their momentum vector facing the forward direction, the resultinggenerated electrons will by conservation of momentum be statisticallyfavored to be emitted in the direction of the gas (i.e., the forwarddirection). This statistically favored probability factor should befurther enhanced by the generated free-electrons feeling the “pull” ofthe pixel EMF field emanating from the interface or junction of thesurface electrode and gaseous layer. For these reasons, the mostcritical or important conversion plate/layer should be on the frontsubstrate.

Integration of PPS Device Collimator—As with any detector, in order torealize the high-resolution imaging potential of the PPS-device, it isnecessary to couple it to a collimator. Generally, the cost of thecollimator should be independent of the radiation sensor, and so itshould make no difference with respect to the collimator whether theradiation sensor is based on PPS, a-Si, photomultiplier, CMOS or CCDtechnology. However, in terms of cost, the PPS could have a unique andsignificant advantage. This is because the physical features andmaterial requirements of a high-resolution collimator could be similarto those for the high-resolution, dual-level plateau, substrate plate(see FIGS. 3 & 4). On a materials basis, both the collimator plate anddual-level substrate plate could be fabricated out of the same metal(e.g. a “workable” tungsten alloy as described above). Thus the“dual-level, micro-segmented plateau” could possibly be designed to alsofunction as a collimator, allowing for shared material cost andauto-alignment.

PPS Image Output—In terms of collecting the image locationinformation/output data for activated PPS pixels, two basic methods areavailable. Since plasma panels are conventionally made of glass and areoptically transparent so as to see the lit pixels (e.g., TV image), aninexpensive CMOS (or CCD) camera (or flat array of such cameras) couldbe located at the back of the plasma panel to record the location ofeach pixel discharge event (see FIG. 6). The camera(s) to be used couldbe low-cost, wide-angle versions of the tiny cameras being integratedinto mobile phones. One such embodiment would consist of a “flat” arrayof integrated CMOS cameras physically positioned as an intermediatelayer between the PPS sensor head and the back-plane module electronics(note that the PPS sensor head is defined as the “bare” plasma panelincluding any dual-level conversion plate). In an alternative embodimentthe module electronics would be placed along the outside perimeter ofthe PPS sensor head. If a camera-based data collection method isemployed, then the PPS back substrate and any associated conversionlayers and dielectric layer would also have to be transparent, which isnot a problem. The second and lower cost, preferred method forcollecting image location/output data, is direct electronic detection.In this embodiment, the detection event circuitry is designed to notonly count the number of detection events, but also to integrate thenumber of events per unit time and to record their spatial location inorder to generate an image of the radiating source. Since each row andcolumn of the PPS device has a current-limiting impedance (e.g.,typically a resistor) to prevent run-away discharges, the location ofeach discharge event can be determined by measuring the row-columnvoltage drop. This latter method requires careful avalanche control(i.e., impedance matching) and fast response times (i.e., rapidelimination of pixel wall charge) to prevent data-loss due to PPS devicedead-time. It is noted that instead of using an external row and columnimpedance, an internal impedance element (e.g., resistor) could bescreen-printed into each pixel to reduce row/column discharge dead-timeand so allow each pixel to be electronically independent of thedischarge state of other pixels on the same electrode. A fabricationprocess for screen-printing a high impedance, series resistor into thecircuit of each discharge cell (i.e., pixel) was developed and employedin the early 1990's by several Japanese companies pursuing color-DC-PDPtechnology for HDTV applications.

Avalanche Control & Response Time—An electron avalanche (i.e., plasmadischarge) has features that need to be controlled: initiation,duration, reset and discharge spreading. Essentially the entire plasmapanel material structure hierarchy, and electronics reactive circuitry(including power supply design), contribute to the above dischargeavalanche characteristics. The specific control parameters include: gasmixture composition and pressure, discharge gap, electrode width andpitch, dielectric constant(s), dielectric cross-sectional thicknesses,dielectric surface and volume resistivity, discharge cell internalimpedance, panel electrode row and column internal impedance, externalreactive circuit impedance including power supply, etc. Sinceminimization of wall charge, along with spreading avalanches and pixeldischarge resetting, can be problematic with AC-PDP's but not withDC-PDP's, the PPS configurations shown in FIGS. 1-4 are primarilyfocused on DC-PDP operation. The two critical metrics required to meetthe goals for avalanche control and fast response time are: (i) limitthe discharge spreading to the spatial resolution of “one cell”, and(ii) limit the cumulative time period of the three avalanche phases(i.e., initiation, duration and reset) to about 1 microsecond (μs) whichwas the time period observed for some of the first modified-PDPradiation sensing devices evaluated. It is noted that the avalancheinitiation and duration phases were typically observed to occur within afew hundred nanoseconds for the modified 1.0 mm electrode pitch PDPdevices studied. However, using a higher resolution panel (e.g. 0.01 to0.1 mm electrode pitch), with a shorter discharge gap, and highervoltage, should lead to a significant reduction of the three avalanchephases to well below 1 μs. Yet, even with a 1 μs pixel response time,each pixel should be capable of recording 1,000,000 counts/sec. Sincethe pixels in a PPS should operate independently and in parallel, a 10μm electrode pitch panel should be capable of recording 6×10¹² countsper second per sq. inch.

Buffer Circuit, Impedance. Data Capture & Noise—In-line impedance is afactor in the initiation, duration and reset phases of an avalanche.Buffer circuitry can prevent feedback of an avalanche across one set ofopposing electrodes from coupling into other sets of electrodes. Abuffer circuit can incorporate in-line impedance and/or discharge-eventsensing. It can also provide gating or synchronization of the powersupply output to an electrode. It is noted that the power supply needsto be under control of the discharge-event signal processing system. Theuse of buffer circuits, with in-line impedance control, is highlyinterrelated with the power supply, gas mixture and electrode systems.These systems must integrally support the initial surge (initiation),and sustain it long enough (duration) to sense a discharge-event, butnot so long as to enable discharge spreading, or electrodedeterioration. The reset phase is for dissipation of gaseous excitedstate species, space/surface charges (internal priming), andre-synchronized electronics. The initiation, duration and reset of adischarge-event occurs on the order of ˜1 μs or less. Thus, capturingand updating/refreshing the PPS image over the module active area at aframe rate of 1 ms (i.e. 1000 fps) should be straight-forward, withframe rates corresponding to 0.1 ms (i.e. 100 μs or 10,000 fps) beingpossible.

FIG. 5 illustrates a micro-sensor circuit 52, which is described in moredetail below. The cross-point within the square receiving a“free-electron” and showing a discharge event (DE) is at theintersection of two opposing electrodes separated by a gap that isfilled with the discharge gas. A power supply (the rectangle with “+”label) is connected through a buffer circuit to an in-line impedance(the cylinder) to one electrode. Across the impedance is connected acircuit to sense voltage drop or current flow in the impedance. Theother electrode is connected to a similar circuit that “returns” toground. For notational purposes, a DE sensed in an X-electrode can bereferred to as DEX, and a DE sensed in the Y-electrode can be referredto as DEY. The “cell” is registered where the DE occurs when DEX and DEYare sensed simultaneously, and can be referred to as a DEXY. Thus, aDEXY is equivalent to a matrix display's “pixel”.

Discharge Event Image Processing—The outputs of the DEX and DEY sensorsare connected to synchronizing circuits (i.e., “DEXY-Former”) thatessentially time-stamp each event referenced to a master clock. TheDEXY-Former provides information to a “Sensitivity Control” circuit thatin turn provides control to the power supply and the DEX and DEYsensors. Thus, the DE processing system is a closely linked, if notclosed loop, system that requires very coordinated integration with bothindividual PPS modules and the global PPS tiled module array system. TheDE processing system also feeds a stream of sequential DEXY frames or“pictures” through a “Link” to a “Sensor Compiler” (57 in FIG. 5). TheDEXY frames or pictures represent the most basic imaging function of thePPS. DEXY frames can be compiled into segments in the time domainindicating intensity of radiation by showing DE spatial and temporaldensity. With appropriate collimators applied to the segmenteddual-level modular array, high resolution source imaging andradioisotope identification of the incident radiation source can beachieved. The radioisotope ID fingerprinting can be further enhanced byprogrammed voltage adjustments at the DE processing level, and systemlevel algorithms in the “Sensor Compiler” and “Detector Processor” (58in FIG. 5).

Discharge Event Camera Augmentation—As previously discussed, the PPS“back” or “second” substrate can be transparent to provide the option ofoptically capturing the DEXYs. Camera requirements, including:resolution, lens, angle of view, focus, shutter/frame speed, CMOS/CCDsensitivity, mounting, etc., need to be evaluated relative tointegrating the camera array back-plane with DE processing andoptimization, including camera synchronization with associated panelelectronics and subsequent Sensor Compiling and Detector Processing. Inparticular, camera augmentation still requires some form of electroniccell (i.e., DE) sensing to be performed to trigger or synchronize thecamera functions.

Signal-to-Noise Ratio—The projected signal-to-noise (S/N) ratio for thePPS should be extremely high, as the plasma panel itself does not employany low voltage, semi-conductor, material structures such astransistors, or any other materials having significant temperatureperformance dependence. Since there are no low voltage bandgap materialspresent that could be thermally populated, there is no reason to employany cryogenics or any other type of active device cooling. It istherefore not likely that a pixel could be thermally induced to lightup, although the possibility exists that under thermal stimulation aninert gaseous atom could collide with the dielectric surface and causean electron charge to transfer to the gas as a free-electron and therebyinitiate a pixel discharge. In general PDP devices are considered tohave the widest operating temperature range of any display device knownand are normally limited not by the plasma panel, but by the thermalrange of the external electronic circuitry. However the S/N ratio, withregard to resolving energy emission lines for purposes of radioisotopeidentification, can always be improved upon by signal enhancement viathe use of larger PPS array collection areas.

Detection Event Circuitry

One embodiment of detection event circuitry that can function ascircuitry 52 shown in FIG. 5 and that detects discharge events is shownin FIG. 7. FIG. 7 is a block diagram illustrating the circuitry attachedto two current-directional electrodes (X and Y) defining a pixelcentroid 84 of a PPS in accordance with one embodiment of the presentinvention for electrically sensing a discharge site. The circuitryattached to the X-axis of the plasma panel includes a power supply 80,an X-Driver 81, a current limit impedance 82 and a DEX pulse detector83. The circuitry attached to the Y-axis of the plasma panel includes aY-Driver 86, a current limit impedance 87 and a DEY pulse detector 85.In either or both axes the current limiting impedances can beimplemented as resistances and/or reactances. In one embodiment, thedrivers, impedances and pulse detectors of FIG. 7 may be active orpassive elements, and may in some combinations be provided by integratedcircuits. Further, in one embodiment, X-Driver 81 and Y-Driver 85 arenot included on an individual electrode basis as is common in commercialPDP video display applications.

The sensing shown in FIG. 7 is initiated through a gamma-ray (orionizing particle) interaction in a PPS conversion layer, generating afree-electron 88 ejected out of the PPS surface interfacing the gas, andinto the high E-field defining the pixel space 84 as previouslydescribed. The mechanism can also be expressed in chronological termsrelative to an avalanche or discharge event (“DE”)—i.e., before, duringor after the avalanche. It is also useful to be able to quantify thesensing ability of the DE in physical terms. In this regard a “bare”plasma panel reference, known as the “discharge margin” (“DM”) voltage,is useful for improving the plasma panel sensitivity. The DM voltagecorrelates to the detection distance sensitivity of the PP-sensor.Specifically, the larger the DM voltage, the greater the apparentsensitivity (or detection distance) of the PP-device with respect to agiven radioactive source. The discharge and/or recovery speed (inmicroseconds), the manufacturing cost, and the collection efficacy areother factors relevant to optimizing embodiments of the presentinvention.

FIG. 8 is a block diagram illustrating detection event processingcircuitry that is attached to the circuitry of FIG. 7 in accordance toan embodiment of the present invention. The detection event processingcircuitry, discussed in more detail below, includes time-stamped DEX's90 and time-stamped DEY's 91, sensitivity control 92, DEXY former 94,imaging function 95, camera 96, and an external computing system 97.

In one embodiment, the electronic sensing and detecting circuitry shownin FIGS. 7 and 8 are involved before, during, and after an avalanche.Power supply 80 biases the electrical sensing mechanisms before theavalanche. The detection electronics (e.g., DEX pulse detector 83, andDEY pulse detector 85) provide the sensing mechanism during anavalanche. The DE processing circuitry (e.g., DEX's 90, DEY's 91,sensitivity control 92, DEXY former 94, imaging function 95, camera 96,and external computing system 97) provides the after-avalanche sensingmechanism.

Power supply 80 in one embodiment is the power supply to the plasmapanel and has one output connected to the X-electrode(s) on one side ofa DC plasma panel. In an embodiment having an AC plasma panel, there isat least one “X” output and one “Y” output from power supply 80. In oneembodiment, power supply 80 can be implemented as multiples of the samecircuit with their separate outputs going to individual electrodes orgroups of electrodes. In one embodiment, power supply 80 is adjustable,under control of the DE processing system described below.

As discussed, a DE equals a gamma-ray (or ionizing particle) sensingevent, therefore the following notation may be used in describingembodiments of the present invention:

DE=discharge event=gamma-ray sensed

DEX=DE sensed in the X-axis (i.e., output column electrode) of theplasma panel

DEY=DE sensed in the Y-axis (i.e., output row electrode) of the plasmapanel

DEXY=DE sensed at a X-Y pixel location of the plasma panel

As described, in embodiments of the present invention, a DE isself-limiting by virtue of an opposing impedance that cuts off thedischarge before a run-away current develops that could vaporize theelectrode(s). The “opposing impedance” in one embodiment is impedance 82and impedance 87 of FIG. 7, which can be resistive in the case of a DCplasma panel. Whenever a DE occurs, the current flowing through theelectrodes and their associated impedances causes a voltage drop acrossthe impedance which terminates the discharge. The momentary voltage dropacross the impedances in the current path of the DE is sensed by DEX andDEY pulse detectors 83 and 85 shown in FIG. 7.

As shown in FIG. 8, the outputs of DEX and DEY pulse detectors 83 and 85are coupled to synchronizing circuits that time-stamp each eventreferenced to a master clock. Therefore, DEX and DEY events with thesame time-stamp are combined to define and store DEXY's in a DEXY former94 circuit. DEXY former 94 provides information to sensitivity control92 circuit that provides feedback information to power supply 80 and DEXand DEY pulse detectors 83 and 85. Feedback control may be implementedfor real-time impedance adjustment and optimization. The DE processingsystem is a closely linked system requiring coordinated designoptimization involving the X and Y impedances, power supply (whether ACor DC), drive waveforms, pulse detectors, and almost every aspect of theplasma panel's configuration and materials selection. The DE circuitryprocessing system in accordance with embodiments of the presentinvention also feeds a stream of sequential DEXY frames or “pictures” toimaging function 95. Because the master clock should be capable ofrunning at a speed high enough to catch DE's sometimes occurringcontinuously, simultaneously and back-to-back, large amounts of raw datacould be generated such that the data might need to be appropriatelyfiltered and/or compressed using known processing methods. In oneembodiment, standard commercial gate arrays, memory chips and other ICcomponents are implemented to meet the DE processing requirements.

In one embodiment, camera 96 is included in the DE processing to performvarious functions. First, if one side of the plasma panel has opticallyclear discharge sites, camera 96 may be used to directly capture DEXY's.Camera 96 is synchronized to sensitivity control 92, DEXY former 94 andimaging function 95. In another embodiment, a second camera function maybe used to capture photodiode (i.e., LED) blinks from current in theelectrode paths. In this embodiment, the photodiodes transform dischargecurrent to light pulses that the camera would capture as DEX and DEY“pictures”, which would further be synchronized and processed in DEXYformer 94 and imaging function 95.

As shown in FIG. 8, in one embodiment the DE processing system feeds astream of sequential DEXY frames or “pictures” from DEXY former 94 toimaging function 95. In one embodiment the DEXY frames are translatedinto a protocol and transport scheme that can be readily connected andprocessed by external computer system 97 using known processingtechniques. A bi-directional bus or network function is represented bythe 4-point arrow structure indicated as the “Synchronization Link” inFIG. 8.

The Stacked PPS Apparatus

A critical figure of merit for a great many material items is theintrinsic device efficiency, although as discussed above, the system orglobal efficiency is usually the more relevant parameter. Optimizationof the basic PPS-device depends in large part upon optimization of theconversion plate/layer, as previously discussed. For example, thethicker the conversion layer, the greater the amount of radiationabsorption (which is good), but the shorter the free-electron transportrange (i.e., the lower the probability of the generated free-electronreaching the gas, which is bad). Whatever the optimum balance may be fora particular application, there will always be a compromise betweenabsorption and electron transport, because these two functions move inopposite directions. However, the efficiency bar can be raisedsignificantly by invoking a system design embodiment that in essenceamounts to a paradigm shift. Namely, the PPS-device, because of itsthin, flat panel structure, allows a vertical-stacking of one PPS-deviceon top of the other, which provides the system designer with a newdegree of freedom. As a result, the PPS-device conversion plateoptimization balance shifts towards using individually thinnerconversion layers (i.e., less absorption), with each plate/layer havingimproved electron transport, and making up for the reduced absorption byvertically stacking more devices on top of each other. FIG. 9 is aperspective view of a vertically-stacked PPS configuration 100. Thissolution is readily affordable since each PPS-device is individuallyvery low in cost. Additionally, this vertical-stack concept applies toall of the PPS structures previously discussed, including: incidentphoton and neutron PPS-detectors, AC and DC structures,columnar-discharge and surface-discharge configurations, etc. Finally,for most properties, the extra degree of freedom associated withvertical stacking, also allows for a variety of innovative hybridstructures, such as: having different spectral response optimizeddevices on top of each other, or different dual-level plateau structureson top of each other, or stacking of different pixel resolution devices,or mixing of columnar-discharge with surface discharge devices, orgamma-ray detectors with neutron detectors, etc. Further, FIG. 9 is aperspective view of another embodiment of the present invention of amonolithically-stacked PPS configuration 110 in which the vertical-stackis monolithic with the back substrate of the first device, serving asthe front substrate of the second device, and so forth, etc.

Additional Details of Embodiments of the Present Invention

Embodiments of the present invention discussed above and below areconfigured for efficient, low-cost detection of ionizing photon andparticle radiation, with a particular emphasis on security applicationssuch as detecting radioactive materials hidden in a moving platform(e.g., FIGS. 5 and 6). The embodiment shown in FIG. 1 is probably themost universal in terms of variety of applications and the lowest costin terms of fabrication. For maximum device efficiency, the two stackedPPS configurations shown in FIG. 9 should offer the highest performance,whereas for radioisotope identification the embodiments shown in FIGS. 3and 4 should provide the greatest selectivity in terms of spectralresolution. However, in terms of overall system design, the tiledPPS-array arrangement shown in FIG. 5 offers the greatest level offlexibility, capability and system value.

With regard to the device operating voltage, in designing a plasma panelto function as a PPS radiation detector in accordance with embodimentsof the present invention, the applied voltage across the dischargeelectrodes should be set at or above the so-called normal display firingvoltage, as there should essentially be no background priming electronswithin the gas gap volume comprising the panel active area. Bymaintaining the discharge electrodes at a well-regulated DC potentialabove the so-called normal display firing voltage, each pixel should beable to react almost instantaneously in generating a localized dischargeevent (i.e., avalanche) upon the occurrence of a radiation-generated,free-electron entering the panel gas-discharge space. However, theelimination of internal priming electrons (so as to raise the deviceoperating voltage above the “normal” display firing voltage),essentially requires that internal wall charge be eliminated or at leastminimized to a very low level. This is probably the single mostimportant design consideration with respect to the various inventionembodiments and to maximizing overall PPS performance as a radiationsensing device. All of the plasma panel sensor head embodimentsdisclosed herein (i.e., FIGS. 1-4) achieve this goal by firsteliminating dielectric material residing above the electrode surface,and then introducing various methods to “bleed” off any remaining chargestored on the dielectric layer immediately in contact with andphysically supporting the surface electrodes, by making this under-layerever so slightly conductive and in direct contact with an appropriatelybiased conductor that can quickly and efficiently drain off any residualcharge.

In terms of practical construction of the various disclosed embodiments,there is a huge volume of publicly available information relating toplasma panel fabrication. Without limiting the Invention, a few of themore basic aspects associated with PPS materials selection, deviceconfiguration, process fabrication and performance optimization areprovided below.

First (Front) Substrate: This is the input side of the plasma panelwhere incident radiation enters. This substrate can be of glass or metalmaterial, or a glass-metal laminate such as a thin metal sheet or foilon the inside surface supported by a thicker glass substrate on theoutside, or even a metal-metal laminate. The top or outside surfacefacing the incident radiation source can be smooth or patterned. In thelatter case, the pattern could be a 3-dimensional structure such as thegrooved-rib pattern shown in FIGS. 3 and 4. If a patterned top surfaceis employed, the patterning can be accomplished by any suitable processincluding: casting, pressing, etching, machining, etc. The bottom orinside surface facing the discharge electrodes and gas should generallybe as smooth as possible so that uniform voltage characteristics can beachieved for each discharge cell. However, for the surface-dischargeconfiguration (SD-PPS), there may be a benefit to the ribbed structurebeing on the inside facing the gas. If the substrate is made of glass,the two most likely materials are 2.8 mm (thickness) Asahi PD200 whichis a glass developed for plasma displays, and 0.5 mm Corning Eagle 2000which is a low-Z, low-density, boro-aluminosilicate glass developed forliquid crystal displays. Commercial display glass substrates, however,continue to be reduced in thickness and this may hold advantages forcertain applications. Of the two above products, the one with the widesttemperature range and highest transparency to low-energy ionizingphotons, such as 20 keV gamma-rays, is the 0.5 mm Corning Eagle 2000,and so this would be the preferred substrate for a PPS with maximumsensitivity to low-energy radiation. For mid-range energy photons thechoice might be PD200 glass; whereas for high-energy ionizing photons,such as 2 MeV gamma-rays, the substrate choice might be a tungsten metalalloy. For slow-neutron detection, the preferred substrate might be agadolinium metal alloy. In general, to maximize the transmission throughthe front substrate and minimize mechanical strain, the front and backsubstrates would benefit by being of identical material composition andthickness, and fabricated from the thinnest material commerciallyavailable and suitable for flat panel processing.

Second (Back) Substrate: This is the output side of the plasma panelwhere the incident radiation not attenuated by the PPS will exit. In avertically-stacked module configuration, such as stack 100 of FIG. 9,the back substrate of the top PPS, faces the front substrate of the nextlower PPS. However in a monolithic-stacked structure, such as structure110 of FIG. 9, the back substrate of the top PPS, also serves as thefront substrate of the next lower PPS, and so use of a glass substrateallows for simplified electrical separation in such monolithic devicessince glass is an insulator. Generally the back substrate should be ofidentical material composition as the front substrate, but if not, itshould have a similar linear coefficient of thermal expansion to avoidundue thermal stress and device warping. Unlike the front substrate,there is no need for the bottom/rear side of the back substrate to havea patterned structure.

Conversion Layers/Plates: One feature of the present invention is theuse of an optimized conversion layer(s)/plate for converting incidentionizing photon (e.g., gamma-ray) and/or particle (e.g., slow-neutronsand fast-neutrons) radiation into free-electrons that can successfullypropagate through the PPS structure to reach the gas volume and initiatea localized pixel avalanche that can be spatially detected, digitallycounted, imaged, and spectroscopically analyzed. Numerous conversionlayer(s) configurations, options, structures, geometries and materialshave been described herein for various radiation detection applications(see FIGS. 1-4), and discussed in terms of specific optimizationstrategies for both individual PPS devices as well as overall systemdesign (see FIGS. 5 and 9).

Electrode Circuitry: To enhance uniform voltage pixel characteristics,the electrodes should generally be as smooth and uniform as possible,which favors fabrication by thin-film processes such as sputtering. Theactual construction method is not pertinent, and terms like X- orY-electrodes, and column- or row-electrodes, are arbitrary and not to belimiting to the Invention. For example, the X- and Y-electrodes could bereversed in orientation and direction in FIGS. 1-4, and the structuresin FIGS. 1 and 2 can be flipped as well, so that the bottom substrate ison top and vice versa. For FIGS. 1-4, the electrode drive circuit may beas simple as a well-regulated, stable DC (or AC) voltage source acrossthe X- and Y-electrodes through as little as one resistor. However, inorder to achieve high performance, most applications will likely imposeadditional considerations for the electronic circuitry as disclosedbelow. For example in FIG. 2 the X- and Y-electrodes are paired togetherin close proximity so that the discharge can be localized. In FIGS. 1-4,current will flow in both electrodes into the drive circuits, which mayalso serve as current (or voltage drop across impedance) sensors.Therefore, the XY “location” of the initial discharge can be determinedmost easily if each electrode has its own drive circuit.

The aforementioned resistor or reactive impedance limits the currentflow during an avalanche and effectively controls the avalancheparameters, which are a function of the resistance (or impedance)together with other panel parameters such as the gas and electrodematerial and geometry. Electrodes may need to be patterned into groupsthat are electrically isolated and driven by separate resistors (orimpedances) to achieve the avalanche characteristics desired.Alternatively, it may be that each electrode needs to have a resistor orother impedance for properly controlled avalanches. In fact, eachindividual discharge cell could have its own resistor or impedance toprovide current sensing that can be processed per circuitry 52 of FIG.5.

As previously discussed, insulating surfaces interfacing the gas mayattract and store charge that inhibits plasma panel device sensitivityand response to incident ionizing radiation. Any spacers (orspacer-barriers) employed or dielectric layers may be implemented aspartially conductive structures that need to have a bias voltageapplied. A bias voltage may be applied on a timing basis, synchronizedto be applied just after an avalanche is detected to remove charge alongthe barrier.

Spacers: There are two types of spacers as shown in FIG. 10: peripheraland internal. Peripheral (or perimeter) spacers are long (i.e., inchesin length) and are positioned close to the panel perimeter, just insidethe seal area. Peripheral spacers are used primarily for holding theseal thickness so as to maintain the proper panel gas gap. Internalspacers are placed within the panel active discharge region and areusually much shorter in length (i.e., can be as small as just a fewpixels long), but can be as long as an entire electrode row or column ina spacer-barrier rib structure (not shown). In general, internal spacerstend to be applied sparingly at only a few necessary positions, justenough to maintain structural integrity in terms of holding a uniformpanel gas gap, and may not be necessary at all for small panel sizes orfor medium size panels in which the internal gas pressure is very closeto the external ambient pressure. For surface-discharge configurations(e.g., SD-PPS and DSD-PPS), the need for internal spacers is reduced asmaintaining a uniform gas gap is not so critical. However, in order toachieve gas gaps on the order of inches in a surface-dischargestructure, the use of a picture-frame, perimeter spacer/seal arrangement(or other such similar configuration) is necessary (see FIG. 10). Thepicture-frame spacer preform itself can be made of any material,although making it conductive would minimize possible problemsassociated with stored charge. Finally, in a barrier-rib type structure,the spacer ribs act as both internal spacers and as barriers todischarge-spreading. In order for the barrier-ribs to avoid wall-chargestorage, they should be partially conductive to “bleed” off storedcharge that tends to get trapped on cell surfaces.

Gas: The “empty space”, or void, in-between the two substrates is filledwith an appropriate gas mixture at an appropriate pressure per theforegoing discussions. The standard methods used to seal PDPs would beemployed to seal the above-described two substrates together (seebelow), thus forming a chamber to contain the gas. The gas most likelywill be a Penning type mixture with an avalanche inhibiter, such as: 98%Xe/2% O₂. If spacer-barriers are used, they must be configured to allowproper gas distribution. To maintain a uniform gas gap without the useof internal spacers in medium size PPS devices, the gas-fill pressureshould be equal to, or very close to, the ambient pressure of ˜700 torr.There are several mechanical means by which this can be accomplishedwhich are beyond the scope of the present invention.

Sealing: The plasma panel seal must maintain a long-term, gas-tight,hermetic barrier under pressure and mechanical stresses. The seal mustalso include a gas-exchange port mechanism as shown in FIG. 10, and beconsistent with setting the proper gas gap in the panel. In general, theseal technology employed for commercial PDPs will be employed in thefabrication of PPS devices.

FIG. 10 illustrates one embodiment of the above described picture-frametype, spacer-seal arrangement on what most likely would be a back platesubstrate 120, and differs significantly from a seal used in a typicalPDP. Back plate 120 includes a gas exhaust and fill hole 124, internalspacers 122, peripheral spacers 128, and a seal 126. Seal 126 includesseal material 127 and picture-frame spacer 129 between the front andback substrate. In most embodiments, the picture-frame spacer seal,peripheral spacers and internal spacers would all have similar height tomaintain a uniform gas gap across the panel. The seal arrangement shownin FIG. 10 is for use primarily in very large gas gap, SD-PPS andDSD-PPS configurations where the gap is on the order of that employed inGM-tubes (e.g. 1-10 mm). In this arrangement, the seal material would becoated as a continuous layer or bead, both above and below thepicture-frame flat surface as shown in FIG. 10.

As described, embodiments of the present invention utilize a plasmapanel structure in conjunction with detection electronics to form aplasma panel based detection device that can be manufactured forrelatively low cost. A number of embodiments of the present inventionare specifically illustrated and/or described herein. However, it willbe appreciated that modifications and variations of the presentinvention are covered by the above teachings and within the purview ofthe appended claims without departing from the spirit and intended scopeof the invention.

1. An ionizing-radiation counting detector comprising: a firstsubstrate; a second substrate generally parallel to said first substrateand forming a gap with said first substrate; at least one spacer todefine said gap; a gas contained within said gap; at least one firstelectrode coupled to said second substrate; at least one secondelectrode electrically coupled to said first electrode and defining atleast one pixel with said first electrode; a first impedance coupled tosaid first electrode; a power supply coupled to at least one of saidelectrodes; a first discharge event detector circuitry coupled to atleast one of said electrodes for detecting a gas discharge countingevent in said electrode; a plurality of pixels, each pixel capable ofoutputting a gas discharge pulse upon interaction withionizing-radiation, wherein each gas discharge pulse is counted ashaving approximately an equal value; and circuitry for detecting if agas discharge pulse is output from the pixels, and for counting each gasdischarge pulse as an individual event.
 2. The ionizing-radiationcounting detector of claim 1, wherein said first electrode is anX-electrode and said second electrode is a Y-electrode.
 3. Theionizing-radiation counting detector of claim 1, further comprising atleast one first driver coupled to said first electrode.
 4. Theionizing-radiation counting detector of claim 1, further comprising: atleast one first driver coupled to said first electrode; and at least onesecond driver coupled to said second electrode.
 5. Theionizing-radiation counting detector of claim 1, wherein the detectorforms a multi-layer detector through vertical stacking.
 6. Theionizing-radiation counting detector of claim 1, wherein said powersupply is a direct current power supply.
 7. The ionizing-radiationcounting detector of claim 1, wherein said power supply is analternating current power supply.
 8. The ionizing-radiation countingdetector of claim 1, wherein said second electrode is coupled to saidfirst substrate.
 9. The ionizing-radiation counting detector of claim 1,further comprising at least one impedance coupled in series with each ofsaid pixels.
 10. The ionizing-radiation counting detector of claim 1,further comprising an internal barrier wall structure between said firstand second substrates to physically isolate said pixels.
 11. Theionizing-radiation counting detector of claim 1, further comprising ahermetic seal coupling said first substrate to said second substrate.12. The ionizing-radiation counting detector of claim 1, wherein saidsecond electrode is coupled to said second substrate.
 13. Theionizing-radiation counting detector of claim 12, wherein a gasdischarge between said first and second electrodes is asurface-discharge shape.
 14. The ionizing-radiation counting detector ofclaim 12, further comprising at least one impedance coupled in serieswith each of said pixels.
 15. The ionizing-radiation counting detectorof claim 12, further comprising an internal barrier wall structurebetween said first and second substrates to physically separate saidpixels.
 16. The ionizing-radiation counting detector of claim 12,wherein the detector forms a multi-layer detector through verticalstacking.
 17. The ionizing-radiation counting detector of claim 12,wherein said gas interacts with said incident ionizing-radiation togenerate a gas discharge counting event.
 18. The ionizing-radiationcounting detector of claim 12, wherein said first substrate is aconversion plate.
 19. The ionizing-radiation counting detector of claim12, wherein said first substrate facing the gas is electricallyconductive.
 20. The ionizing-radiation counting detector of claim 12,wherein at least one third electrode is physically coupled to said firstsubstrate.
 21. The ionizing-radiation counting detector of claim 20,wherein at least one fourth electrode is physically coupled to saidfirst substrate and electrically coupled to said third electrode, anddefining at least one pixel with said third electrode.
 22. Theionizing-radiation counting detector of claim 12, further comprising aconversion layer coupled to said first substrate and facing the gas. 23.The ionizing-radiation counting detector of claim 22, wherein saidconversion layer converts incident photons to free-electrons emittedinto the gas.
 24. The ionizing-radiation counting detector of claim 22,wherein said conversion layer converts incident ionizing-particles tofree-electrons emitted into the gas.
 25. The ionizing-radiation countingdetector of claim 22, further comprising at least one impedance coupledin series with each of said pixels.
 26. A method of detectingionizing-radiation based on a counting of gas discharge eventscomprising: receiving ionizing-radiation in a gas contained within a gapbetween a first substrate and a second substrate of a surface-dischargeplasma panel; creating at least one free-electron within the gas inresponse to the received ionizing-radiation; causing a gas-dischargeevent at a pixel site of an X-electrode and a Y-electrode on the secondsubstrate of the surface-discharge plasma panel; and counting aplurality of said events with pulse detection circuitry coupled to thesurface-discharge electrodes, wherein each of said events is counted ashaving approximately an equal value.